What is SMTP, POP3 & IMAP & How does it Work?

In the realm of email communication, acronyms like SMTP, POP3, and IMAP are frequently mentioned. But what do they actually mean, and how do they contribute to the seamless transfer of messages across the internet? In this blog post, we will unravel the mysteries behind SMTP, POP3, and IMAP, exploring their functions, differences, and how they work together to ensure efficient email delivery and management.

SMTP, POP3 & IMAP: Have you ever wondered about the technology behind the Emails sent to you every day? It’s actually several simple technologies working together, technologies that we take for granted when we compose a message and click send. On the back of each email, there is a powerful engine called the email server, this email server actually pushes the emails through the internet. All of us use them, but little did we know how does an email server work?

To drive a car you need not have to be an expert to differentiate a carburetor from a tailpipe, but when your car breaks down, this little over-the-top knowledge helps. Email rarely crashes but you should know how Pip’s mail reaches Estella across the globe in a moment.


Every email message is a text file plus the attachments. Like other internet data, an email is also broken into smaller packets. As soon as you click the send button, all these packets are uploaded to a central computer (the email server) that hosts the email service.

This email service now comes into action and then starts relaying these packets through the internet to that server that holds your recipient’s email service. Now the recipient’s mail server looks for its email address, locates it, and places the email within the inbox. This email client reassembles the packets into a complete mail. What happens next? Your recipient logs into her/his account and downloads the email. Job Done! See the magic of SMTP, POP3 & IMAP in action.

SMTP, POP3 & IMAP all three are TCP/IP protocols that are used for mail delivery. Just for your reference, each protocol is actually a specific set of communication rules between various computers.

SMTP (Simple Mail Transfer Protocol)

SMTP is the backbone of email communication. It is a protocol for sending outgoing emails from the sender’s email client or server to the recipient’s mail server. When you compose and send an email, your email client communicates with the SMTP server, which acts as a mail transfer agent responsible for relaying your message to the appropriate destination. SMTP service ensures the email is formatted correctly, addressed, and delivered securely.

SMTP Protocol:

  1. End-to-end
  2. Store-and-forward

The end-to-end method is a data communication protocol in which data is sent directly from the source to the destination without any intermediate nodes inspecting or modifying the content. The end devices, such as computers or servers, ensure data reaches its destination correctly and efficiently.

In the end-to-end method, data packets are transmitted through the network. Each intermediate node in the path forwards the packets without any active processing. The intelligence and decision-making are primarily handled by the source and destination devices, which allows for a simpler and more efficient network design.

The essential advantage of the end-to-end method is its simplicity and minimal overhead in the intermediate nodes. This reduces the chances of bottlenecks or delays caused by complex processing at intermediate points. Therefore, the end-to-end method is suitable for real-time applications that require low latency and reliable data delivery.

The store-and-forward method is a data communication protocol in which data packets are received and stored temporarily at each intermediate node or network device before being forwarded to the next destination. Each intermediate node inspects the data packets, and in some cases, it may perform certain processing or checks before forwarding the packets.

Unlike the end-to-end method, which relies on the source and destination devices for most decision-making, the store-and-forward method allows intermediate nodes to participate in the data transmission process actively. This enables the nodes to perform error checking, data filtering, protocol conversion, and congestion control tasks. One key advantage of the store-and-forward method is its ability to provide more excellent reliability and data integrity.

Advantages of SMTP

  1. Reliable Email Delivery: SMTP ensures reliable and efficient email delivery. It verifies the recipient’s address and formats the email correctly, ensuring that messages reach their intended recipients.
  2. Widely Supported: SMTP is a widely adopted protocol for sending emails. Virtually all email clients and servers support SMTP, making it a universal choice for email communication.
  3. Straightforward Protocol: SMTP’s simplicity contributes to its ease of implementation. It has a clear set of communication rules, which makes it relatively straightforward to understand and work with.
  4. Multiple Delivery Paths: SMTP supports multiple delivery paths, allowing emails to be relayed through multiple servers if necessary. This redundancy helps in case one server is unavailable or experiencing issues.
  5. Compatibility with Authentication: SMTP supports various authentication mechanisms, enhancing security by ensuring that only authorized users can send emails through the server.

Disadvantages of SMTP

  1. Limited Security: SMTP was designed for message delivery, not ensuring message security. Emails sent using SMTP can be intercepted and read, requiring additional security measures like encryption.
  2. No Message Synchronization: SMTP does not synchronize messages across devices or servers. Once the email is delivered, SMTP’s job is done, and the email client/server is responsible for managing messages and their synchronization.
  3. No Error Handling for Recipients: SMTP doesn’t provide mechanisms for handling errors on the recipient’s end. The sender might not receive immediate notifications if an email is sent to an incorrect or nonexistent address the sender might not receive immediate notifications.

POP3 (Post Office Protocol version 3)

POP3 is a protocol for retrieving emails from a remote mail server to the recipient’s device. When you configure your email client with POP3 settings, it connects to the POP3 server to download your incoming emails. POP3 allows you to access and manage your emails locally, as it typically downloads the messages from the server and removes them from the server by default. However, some configurations allow you to choose whether to leave a copy on the server or delete it after downloading.

Despite several enhancements since its inception in the late 1980s, POP3 continues to be widely used due to its simplicity. Its popularity also stems from its efficient email retrieval process, minimizing errors.

This protocol is particularly well-suited for situations where users need to access their emails offline and use a designated device for retrieval. Additionally, POP3 proves beneficial for sending and storing bulk email messages.

However, it is essential to note that POP3 is not designed to support email manipulation or server synchronization, as its primary purpose is to download emails to the client and subsequently delete them from the server. The more advanced and intricate Internet Message Access Protocol (IMAP) is preferred for such use cases.

IMAP not only allows polling of existing connections for newly arrived messages but also supports multiple folders on the server, capabilities that are not present in POP3.

Advantages of POP3

  1. Offline Access: One of the main advantages of POP3 is that it allows offline email access. Once emails are downloaded to the local device, users can read and manage them without an internet connection.
  2. Email Storage Control: By default, POP3 downloads emails from the server and removes them. This can help users manage their server storage space, as emails are stored locally.
  3. Simple Configuration: Setting up a POP3 email client is relatively straightforward. Users need to configure the email client to connect to the POP3 server, and they can start receiving emails.
  4. Reduced Server Load: Since emails are typically downloaded to the client and removed from the server, the server load is reduced compared to protocols like IMAP, where messages are stored on the server.
  5. Useful for Low-Bandwidth Connections: POP3 can be useful in areas with limited internet bandwidth, as emails are downloaded and can be accessed offline, reducing the need for constant data usage.

Disadvantages of POP3

  1. Limited Synchronization: POP3 lacks real-time synchronization. Changes made on one device (e.g., marking an email as read) won’t be reflected on other devices, making it less suitable for users who need consistent access across devices.
  2. Message Access Limited to One Device: Since emails are downloaded to a specific device, accessing emails from multiple devices becomes challenging. This limitation is not suitable for users who switch between devices frequently.
  3. Limited Message Storage: Local storage limitations can impact the number of emails stored on the client device. If not appropriately managed, users might run out of email storage space.
  4. Risk of Data Loss: If emails are only stored locally and not backed up, there’s a risk of data loss due to device malfunction, theft, or accidental deletion.

IMAP (Internet Message Access Protocol)

IMAP is another protocol for retrieving emails but with a different approach. IMAP allows users to access and manage their emails directly on the mail server. When you access your mailbox through IMAP, your email client synchronizes with the server, giving you a real-time view of your emails and folders. This means that changes made on your email client (such as marking emails as read, moving them to folders, or deleting them) are reflected on the server and vice versa. IMAP is particularly useful when accessing emails from multiple devices since all changes are synchronized.

IMAP was designed with a specific purpose, offering a more flexible way for users to access their mailboxes. It operates in three modes: online, offline, and disconnected. However, the offline and disconnected modes are of particular interest to most users of the protocol. Here are the key features of the IMAP protocol:

  1. Access and retrieve mail from remote server: Users can access their mail from the remote server while storing the messages on the server.
  2. Set message flags: Users can set flags to track which messages they have already seen.
  3. Manage multiple mailboxes: IMAP allows users to manage multiple mailboxes and transfer messages between them, organizing them into various categories, which is especially useful for those working on different projects.
  4. Determine information before downloading: IMAP enables users to decide whether to retrieve specific mail before downloading it from the server.
  5. Downloads a portion of a message: Users can download only a specific portion, such as one body part from a mime-multi part. This feature is helpful when dealing with large multimedia files within a short-text element of a message.
  6. Organize emails on the server: Unlike POP3, IMAP allows users to organize their emails. They can create, delete, or rename mailboxes on the server based on their requirements.
  7. Search: Users can perform searches within the contents of their emails.
  8. Check email header: IMAP allows users to check email headers before downloading the complete messages.
  9. Create hierarchy: Users can create folders and organize their emails hierarchically.

IMAP General Operation: IMAP is a client-server protocol similar to POP3 and many other TCP/IP application protocols. IMAP4 functions when installed on the server where user mailboxes are located. Unlike POP3, which does not require the same physical server for SMTP services, IMAP requires access to the mailbox for both incoming mail via SMTP and retrieval/modifications via IMAP.

The IMAP protocol utilizes the Transmission Control Protocol (TCP) for communication, ensuring orderly and reliable data delivery. IMAP4 listens on a well-known port, port number 143, to handle incoming connection requests from IMAP4 clients.

Advantages of IMAP

  1. Real-Time Synchronization: IMAP provides real-time synchronization between the email server and the client. Changes made on one device (such as reading, deleting, or moving emails) are reflected across all devices accessing the same account.
  2. Cross-Device Access: IMAP allows users to access their emails from multiple devices, such as smartphones, tablets, and computers. This is particularly useful for users who switch between devices frequently.
  3. Server-Side Storage: IMAP stores emails on the server, meaning users can access their email history from any device. This is especially valuable in cases of device loss or when switching devices.
  4. Organizational Features: IMAP supports hierarchical folder structures on the server, enabling users to organize their emails into different folders and categories. This organizational flexibility is crucial for users with large volumes of emails.
  5. Advanced Search Capabilities: IMAP provides robust search functionality that allows users to quickly find specific emails based on various criteria, such as sender, subject, date, and keywords.
  6. Message Flags: IMAP allows users to mark emails with flags, which can indicate different statuses like “read,” “unread,” “flagged,” etc. These flags are synchronized across devices.

Disadvantages of IMAP

  1. Server Dependence: IMAP relies on a constant internet connection to synchronize emails. Offline access to messages can be limited compared to POP3, which downloads emails to the local device.
  2. Storage Limitations: The server storage space is a concern, especially for users with large email attachments. Exceeding storage limits may require users to manage their mailbox more actively.
  3. Complexity: IMAP’s advanced features can make its setup and configuration more complex than POP3. Some users may find it challenging to configure their email clients properly.
  4. Server Resource Usage: IMAP requires more server resources than POP3, especially in cases of frequent synchronization and larger mailbox sizes.

Difference between SMTP, POP3 & IMAP

This table will help you understand the key difference between SMTP, POP3 & IMAP.

AspectSMTP (Simple Mail Transfer Protocol)POP3 (Post Office Protocol version 3)IMAP (Internet Message Access Protocol)
FunctionOutgoing email deliveryIncoming email retrievalIncoming email retrieval and management
Email AccessN/AOffline access to downloaded emailsReal-time synchronization across devices
Message StorageN/ALocal storage on the client deviceServer-side storage
Message DeletionN/ARemoves messages from the serverOptionally removes or keeps messages
Folder ManagementN/AN/AHierarchical folder organization on the server
Message FlagsN/AN/AFlags and read/unread synchronization
Search CapabilitiesN/ALimited search featuresAdvanced search functionality
CollaborationN/AN/ASupports collaboration and shared access
Offline AccessN/AYes, with downloaded emailsLimited offline access to cached messages
Security ConsiderationsEnsures proper email formattingPotential for unencrypted communicationSupports secure communication and encryption
Server LoadRelays emails to the recipient’s serverN/ASynchronization can load the server
Configuration ComplexityRelatively straightforwardRelatively straightforwardMay be more complex due to synchronization
Cross-Device AccessLimited to sending deviceLimited to downloading deviceReal-time access from multiple devices
Message SynchronizationN/AN/AReal-time synchronization across devices

How SMTP, POP3, and IMAP Work Together

The collaboration of these protocols ensures a seamless email experience:

  1. Outgoing Email Delivery: When you compose and send an email using your email client or webmail interface, SMTP comes into action. Your email client communicates with an SMTP server, usually provided by your email service provider or your organization’s server. The SMTP server is a mail transfer agent and manages the entire delivery process.

First, your email client establishes a connection with the SMTP server. It authenticates your credentials to ensure you have the authorization to send emails. Once authenticated, you provide the recipient’s email address, the subject, and the body of the message.

The SMTP server performs various checks, such as verifying the recipient’s address, ensuring proper formatting, and checking for potential spam content. It then encapsulates your email into a structured format, known as the Multipurpose Internet Mail Extensions (MIME) format, which includes the necessary headers, message body, and attachments.

Next, the SMTP server routes the email to the recipient’s mail server. This process might involve multiple intermediate servers or relays, each responsible for forwarding the message to the next hop until it reaches the final destination. The SMTP server communicates with the recipient’s mail server using the recipient’s domain’s DNS (Domain Name System) records to determine the server’s address.

Once the recipient’s mail server receives the email, it performs further checks and validations, such as spam filtering and virus scanning. Finally, the email is delivered to the recipient’s inbox.

  1. Incoming Email Retrieval: When receiving emails, you can configure your email client with either POP3 or IMAP settings.

If you opt for POP3, your email client connects to the POP3 server associated with your email account. It authenticates your credentials and establishes a secure connection. Once connected, the POP3 server lists your emails stored on the server. Your email client retrieves these emails by downloading them to your local device. By default, POP3 removes the emails from the server, but you can configure it to keep copies on the server if desired.

In contrast, your email client establishes a synchronized connection with the IMAP server if you choose IMAP. This connection lets you view and manage your emails directly on the server, maintaining consistency across multiple devices. When you open your email client, it syncs with the IMAP server, fetching the latest updates, including new messages, read/unread status, and folder organization. Changes made on your email client are reflected on the server, and vice versa, ensuring that your emails are always up to date.

It’s important to note that both POP3 and IMAP can coexist for the same email account. This allows you to choose the preferred email access method based on your needs. For example, you might use POP3 on your personal computer to store emails locally while using IMAP on your smartphone to access emails on the go.

By working in tandem, SMTP, POP3, and IMAP enable the seamless flow of email communication. SMTP ensures the successful delivery of outgoing emails, while POP3 and IMAP facilitate the retrieval and management of incoming messages, catering to different preferences and requirements.

Understanding how these protocols collaborate empowers users to make informed choices in configuring their email clients and optimizing their email communication experience. Whether composing and sending emails or accessing and organizing incoming messages, SMTP, POP3, and IMAP work together to ensure efficient and reliable email communication across the globe.

SMTP (Simple Mail Transfer Protocol), POP3 (Post Office Protocol version 3), and IMAP (Internet Message Access Protocol) are integral components of the email ecosystem, working together to ensure efficient and reliable communication. Each protocol plays a specific role in the process, and understanding their collaboration provides valuable insights into the mechanisms behind email delivery and management.

SMTP serves as the foundation for sending emails. It establishes a connection between the sender’s email client or server and the recipient’s mail server. When you compose an email and hit the send button, your email client communicates with the SMTP server to initiate the transfer. SMTP takes care of verifying the recipient’s address, formatting the email correctly, and ensuring secure transmission. It acts as a mail transfer agent, relaying the email through a series of hops if needed, until it reaches the recipient’s mail server.

On the other hand, POP3 and IMAP come into play when you want to retrieve and manage your incoming emails. POP3 focuses on downloading emails from the remote mail server to your device. When you configure your email client with POP3 settings, it connects to the POP3 server, authenticates your credentials, and retrieves the emails. By default, POP3 removes the messages from the server, but you can choose to leave copies if desired.

In contrast, IMAP allows you to access and manage your emails directly on the mail server. Configuring your email client with IMAP settings establishes a synchronized connection with the IMAP server. This connection enables real-time access to your mailbox, ensuring that changes made on your email client (such as reading, moving, or deleting emails) are reflected on the server and all devices. IMAP provides a consistent view of your inbox, regardless of the device you use to access your emails.

The collaboration between SMTP, POP3, and IMAP ensures a seamless email experience. SMTP handles the sending of emails, while POP3 and IMAP take care of retrieving and managing incoming messages. This division of labor allows users to send and receive emails effortlessly, with options for local storage (POP3) or server-based access (IMAP).


In conclusion, it explains the crucial roles of SMTP, POP3, and IMAP in email communication. Though often taken for granted, these protocols work collaboratively to ensure efficient and reliable transfer of emails. SMTP is the backbone for outgoing email delivery, ensuring proper formatting and secure transmission. POP3 enables offline access by downloading emails to the client device, while IMAP provides real-time synchronization across devices and server-side storage. The blog highlights how the protocols cooperate to offer a seamless email experience, allowing users to send, receive, and manage their messages effortlessly. Understanding this collaboration empowers users to configure their email clients optimally and make informed choices regarding their email communication.

Increase your Prospect's Engagement with Multichannel Outreach

Loved it? Spread it across!
Scroll to Top