Why Do Emails Bounce | How To Lower Email Bounce Rate?

You hit send on an important email campaign, confident your message will reach 100 prospects. Hours later, you check your inbox and find 15 bounce-back notifications. Your carefully crafted emails never made it. What went wrong?

Email bounces are frustrating, but they’re not random. Every bounced email returns with specific error codes that explain exactly why delivery failed. Understanding these codes, and the root causes behind them, is the first step to improving your email deliverability.

In this guide, you’ll learn the mechanics of email bounces, the difference between hard and soft bounces, how to read SMTP error codes, and what each bounce type means for your campaigns.

Email bounce: What Is It?

An email bounce occurs when your email fails to reach its intended recipient and is returned to you by the recipient’s mail server. When this happens, you receive an automated message called a Non-Delivery Report (NDR) or bounce-back message.

Think of it like sending a letter through postal mail. If the address is wrong, the post office returns it to you with a reason: “Address Unknown,” “Moved, Left No Address,” or “Mailbox Full.” Email bounces work the same way, except the “post office” is an email server, and the reasons are communicated through SMTP error codes.

Key Components of a Bounce:

• Non-Delivery Report (NDR): Automated message explaining why delivery failed
• SMTP Error Code: 3-digit numerical code (e.g., 550, 421)
• Bounce Reason: Human-readable explanation (e.g., “Mailbox full”)

What Causes an Email to Bounce Back?

When you send an email, your email server (SMTP client) communicates with the recipient’s email server (SMTP server) through a “handshake” process:

1. Connection: Your server connects to the recipient’s server
2. Authentication: Your server identifies itself (HELO/EHLO command)
3. Mail Transfer: Your server sends the email
4. Response: The recipient’s server accepts or rejects the email

If the recipient’s server rejects your email at any stage, it sends back an SMTP error code explaining why. Your server then generates an NDR and sends it to you.

Example SMTP Handshake:

S: 220 mail.example.com ESMTP
C: EHLO sender.com
S: 250-mail.example.com
C: MAIL FROM:[email protected]
S: 250 OK
C: RCPT TO:[email protected]
S: 550 5.1.1 User unknown

In this example, the recipient’s server rejected the email at the RCPT TO stage because the email address doesn’t exist (550 5.1.1).

There are many reasons for an email to bounce back. Before we can figure out how to resolve the issue of high bounce rates, we need to look into what causes an email not to reach its senders. There are five primary reasons for an email to bounce back. 

• Full inboxes
• Unresponsive servers
• Emails sent to false email addresses
• Emails sent to email addresses that no longer exist 
• Your email address is on the ISP block list. 

These unsent emails fall under three categories of email bounces. 

1. Soft bounce emails
2. Hard bounce emails
3. Blocklisted emails

Let us see what these three types of email bounces entail and how to troubleshoot them. 

Soft bounce emails

If the email receiver server rejects your email temporarily, you have a case of soft bounce emails. Soft bounce emails are usually temporary, and you can re-send the email after a week or two has passed. Here are some reasons for a soft bounce:

Soft Bounce Cause	SMTP Code	What it Means	Action
Full Inbox	452 / 4.2.2	Recipient’s mailbox storage exceeded	Retry after 24-72 hours
Email Too Large	452 / 4.3.1	Attachment exceeds size limit	Reduce file size or use links
Server Temporarily Down	421 / 4.4.1	Recipient’s server offline/maintenance	Retry automatically
Greylisted	451 / 4.7.1	Temporary spam prevention delay	Server retries after delay
Auto-Reply	450	Out-of-office or vacation responder	Not a real bounce, ignore

• A full inbox: An email server will give users limited email space. Once a user has used that space, there will be no space to accept emails. In this case, your email will bounce back until there’s free space.

• Email size: Multimedia-heavy emails may bounce back because of filters put in place by the ISPs or the users themselves. The attachment size limitations depend on the ISP 

• Autoreply: This feature is used when the email recipients are not reachable for extended periods. An auto-reply will make an email bounce back.

• Unresponsive server: Sometimes, email servers can go into overload and crash. Unresponsive servers can also be a result of maintenance. 

Hard bounce emails 

A hard bounce means that your email won’t go through at all. There are several reasons for it.

Hard Bounce Cause	SMTP Code	What It Means	Action
Invalid Email Address	550 / 5.1.1	Address doesn’t exist or misspelled	Remove from list immediately
Domain Doesn’t Exist	550 / 5.1.2	@domain.com is invalid/expired	Remove from list immediately
Recipient Blocked You	550 / 5.7.1	Recipient marked you as spam	Remove from list immediately
Disposable Email	550 / 5.1.1	Temporary/burner email expired	Remove from list immediately
Policy Rejection	550 / 5.7.1	Violates recipient’s email policy	Review content, remove from list

• Misspelled email addresses: This can cause an email to a hard bounce. Make sure to avoid spelling mistakes while typing out email addresses. 

• Disposable emails: With the emergence of services like temporary emails or disposable emails, it’s becoming easier than ever to have your emails not delivered to any address. 

• Fake email addresses: Sometimes, people give out fake ones when asked for them. The best way to avoid this is by verifying through double opt-ins.

• Blocked: If the recipient blocks your email address, then there is no way you can get your email to them. 

The best way to troubleshoot this issue is to delete these hard-bounced email addresses. Too many hard bounces can get your email address blocklisted. 

Blocklisted emails

Spam is becoming more and more prevalent each day. Therefore, ESPs (email service providers) create email blocklists to filter out harmful email content like malware and spam. Make sure your email address doesn’t end up in the ESPs blocklist. Let us look into how an email gets into an ESPs blocklist in the first place. 

• A surge in email volume: If you scale up your email send list too suddenly, it can point towards buying an email list. This mirrors certain spam practices and can land you in the ESP blocklist. 

• High bounce rates: You will get high bounce rates if you keep using the same old email send list without clearing the blocklisted emails. 

• Spam complaints: If your email address receives too many spam complaints, your email might end up in the ESP blocklist. 

Being on a blocklist can hamper your campaign adversely. There is a high chance that your email doesn’t reach the inbox of anyone in your campaign email list. But fret not; here are a few email hygiene practices to avoid ending up on anyone’s blocklist. 

1. Don’t use spammy language while sending your email. Think of ‘LIMITED PERIOD OFFER’ or ‘BUY NOW OR REGRET FOREVER’ in caps followed by overuse of emojis and exclamation marks may trigger spam filters. 

2. Remove dead-ended emails from your campaign’s email list. 

3. Change your IP if you have been blocklisted. Changing providers can keep you off the ESP’s blocklists. 

4. Never ever buy an email list. It may seem like an easy and shortcut way to get a ready-made campaign email list, but you can’t judge the quality of the email list, and it is the fastest way to get your IP address onto ESP’s blocklists. 

If you find yourself in a sticky situation where you are on a blocklist, and you need to get off of it because, say, you can’t change ISP, then there are some ways to get off the blocklists.

• First off, give it time. Most blocklists will remove a blocked email after a certain amount of time. 

• Most blocklists can be accessed publicly. Contact the blocklist website holder if you find your email address on one of these lists. A well-formatted email can do very well to persuade the blocklist provider to remove your email from their blocklist. 

Email bounce is more common because Internet Service Providers (ISPs) and individuals are making spam filters more diverse. Therefore, if your email contains a sentence that is recognized by the spam filter, it will not reach your leads. Your email bounce rate should stay below 3%. An email bounce rate of anything above 2 percent should be investigated. 

Common SMTP Codes for Blocklists:

• 550 5.7.1: “Your IP is blacklisted”
• 554 5.7.1: “Sender domain blocked due to spam complaints”
• 421 4.7.0: “Temporarily rejected due to IP reputation”

12 Common Reasons Why Emails Bounce

Here are the 12 most common reasons your emails bounce back:

Hard Bounce Reasons (Permanent)

1. Invalid or Non-Existent Email Address
– The email address is misspelled, fake, or never existed
– SMTP Code: 550 5.1.1

2. Domain Name Doesn’t Exist
– The @domain.com part is invalid or expired
– SMTP Code: 550 5.1.2

3. Recipient Blocked Your Email Address
– Recipient marked you as spam or manually blocked you
– SMTP Code: 550 5.7.1

4. Disposable/Temporary Email Expired
– Burner emails that self-destruct after hours/days
– SMTP Code: 550 5.1.1

5. Email Policy Violation
– Your email violates recipient’s content filters
– SMTP Code: 550 5.7.1

6. DNS/MX Record Issues
– Recipient’s domain DNS is misconfigured
– SMTP Code: 550 5.4.4

Soft Bounce Reasons (Temporary)

7. Recipient’s Inbox is Full
– Mailbox has exceeded storage limit
– SMTP Code: 452 4.2.2

8. Email Size Too Large
– Attachments exceed recipient’s size limit (usually 25MB)
– SMTP Code: 552 5.3.4

9. Server Temporarily Offline
– Recipient’s server is down for maintenance or overloaded
– SMTP Code: 421 4.4.1

10. Greylisting (Spam Prevention)
– Temporary delay to verify sender legitimacy
– SMTP Code: 451 4.7.1

11. Spam Filter Triggered
– Content triggered recipient’s spam filter
– SMTP Code: 550 5.7.1 (sometimes)

12. Authentication Failure (SPF/DKIM/DMARC)
– Your domain lacks proper email authentication
– SMTP Code: 550 5.7.1

Understanding SMTP Error Codes

Every bounce-back email includes an SMTP (Simple Mail Transfer Protocol) error code that explains why delivery failed. Understanding these codes helps you diagnose and fix bounce issues faster.

SMTP Code Format:

SMTP codes follow a 3-digit format: XYZ

• First Digit (X): Response category
• 2xx: Success (email delivered)
• 4xx: Temporary failure (soft bounce)
• 5xx: Permanent failure (hard bounce)
• Second Digit (Y): Response subcategory
• X.0.X: General status
• X.1.X: Addressing issues (invalid address, domain)
• X.2.X: Mailbox issues (full, disabled)
• X.3.X: Mail system issues (size, policy)
• X.4.X: Network issues (connection, routing)
• X.5.X: Protocol issues (syntax, authentication)
• X.7.X: Security issues (blocked, spam, policy)
• Third Digit (Z): Specific error

Example: 550 5.1.1

• 5: Permanent failure (hard bounce)
• .1: Addressing issue
• .1: Mailbox doesn’t exist

Common SMTP Codes Quick Reference:

SMTP Code	Type	Meaning
421	Soft	Server temporarily unavailable
450	Soft	Mailbox temporarily unavailable
451	Soft	Greylisted or local error
452	Soft	Mailbox full or storage exceeded
550	Hard	Mailbox not found / Invalid address
551	Hard	User not local / Relay denied
552	Hard	Email size exceeds storage limits
553	Hard	Invalid mailbox format
554	Hard	Transaction failed / Blocked

Enhanced SMTP Codes:

Modern email servers use Enhanced SMTP codes with an additional decimal format: X.Y.Z

Example: 5.1.1 (mailbox doesn’t exist)

• 5: Permanent failure
• .1: Addressing issue
• .1: Mailbox unknown

How to Read Your NDR (Non-Delivery Report):

When you receive a bounce-back email, look for:

1. SMTP Error Code: The 3-digit number (e.g., 550)
2. Enhanced Code: The X.Y.Z format (e.g., 5.1.1)
3. Human-Readable Message: Plain text explanation
4. Original Email: Copy of your sent email

Example NDR:

From: [email protected] To: [email protected] Subject: Undelivered Mail Returned to Sender

This is the mail system at host mail.example.com.

I’m sorry to have to inform you that your message could not be delivered to one or more recipients. It’s attached below.

For further assistance, please send mail to postmaster.

[email protected]: host mail.domain.com[192.0.2.1] said: 550 5.1.1 [email protected]: Recipient address rejected: User unknown in local recipient table (in reply to RCPT TO command)

How to Diagnose Why YOUR Emails Bounced

If your emails are bouncing, follow this diagnostic checklist to identify the root cause:

Step 1: Check the SMTP Error Code

• Look at the bounce-back email’s error code (e.g., 550, 421)
• Reference the SMTP code table above
• Determine if it’s a hard bounce (5xx) or soft bounce (4xx)

Step 2: Verify the Email Address

• Check for typos (e.g., @gmial.com instead of @gmail.com)
• Verify the domain exists (use MXToolbox.com)
• Confirm the mailbox exists (use email verification tools)

Step 3: Review Your Email Content

• Check for spam trigger words (“FREE”, “URGENT”, “ACT NOW”)
• Verify link-to-text ratio (too many links = spam)
• Confirm proper authentication (SPF, DKIM, DMARC)

Step 4: Check Your Sender Reputation

• Use tools like Google Postmaster, Microsoft SNDS
• Check if your IP is blocklisted (MXToolbox, Spamhaus)
• Review complaint rates (should be <0.1%)

Step 5: Analyze Bounce Patterns

• Soft bounces to specific domains? → Server issue on their end
• Hard bounces across different domains? → List quality issue
• Sudden spike in bounces? → Reputation or authentication issue

When to Retry vs Remove:

Bounce Type	Wait Time	Max Retries	Action if Still Bouncing
Soft Bounce (Inbox Full)	24-48 hours	3-5 times	Remove after 72 hours
Soft Bounce (Server Down)	2-4 hours	10-15 times	Remove after 7 days
Hard Bounce	Don’t retry	0	Remove immediately
Greylisted	15 min – 1 hour	Automatic	No action needed
Blocklisted	N/A	0	Fix reputation, change IP

Quick Fixes for Common Bounce Issues

While understanding bounce causes is critical, reducing bounces requires systematic tactics:

Quick Fixes:

• For Hard Bounces: Remove invalid addresses immediately using email verification tools
• For Soft Bounces: Wait 24-72 hours and retry before removing
• For Blocklists: Use MXToolbox to check status and request delisting
• For Authentication Issues: Set up SPF, DKIM, and DMARC records

For a complete guide to reducing email bounce rates with step-by-step tactics, read our detailed article: [How to Reduce Email Bounce Rate](link to existing blog).

A lot of the solutions discussed above can be a good springboard for your effort to lower your email bounce rate. However, this is not an exhaustive list. Employing an ABP (Account-Based Prospecting) style email campaign will address most of your issues about not having a high email bounce rate. ABP-style email campaigns also, on average, bring in more responses than a normal cold-email campaign. But do remember that when using ABP, if either prospect’s email bounces due to “email blocked,” the entire list of emails might probably bounce. Consider this when creating a strategy for sending emails.

SmartReach.io specializes in cold email campaigns. It has special features like spam test reports, email validations, campaign soft start etc that directly or indirectly assist in lowering your email bounce. Its advanced personalization features also prevent the triggering of ESP’s spam filters that might get you on their blocklists.

There is a science and art to understanding the intricacies of a mass-deployed email campaign. With so many moving parts in action, it can be hard to keep track of certain things. SmartReach ensures that your emails reach the primary inboxes of the highest-quality leads or prospects. Our specialty software also makes it easy to replicate the human tone that is necessary to connect to a lead on a personal level. This level of care and personalization allows the prospect to convert into a sale. 

To sum it up

Any email campaign bounce rate over 3% should be investigated.

A successful campaign is possible if you avoid the common traps of spam filters and avoid spammy behavior.

Most organizations nowadays use personalized systems to craft successful email campaigns. It is known as ABP (Account Based Prospecting)-an emailing system. This results in personalized emails to individuals of a particular company that is part of your ICP; as a result, a higher email response rate!

Ultimately, you should try your best to keep your email bounce rate low. We acknowledge that complete elimination of the bounce rate is not possible, but the methods mentioned above should be a good starting point. Remember these few key pointers:

• Rectify server and IP issues as soon as they happen. 
• Organically grow your leads list instead of buying premade lists. 
• Honor opt-out requests from your emails. 
• Clean up your email list. Remove emails that have hard-bounced in the past. 
• Send a high-quality email that doesn’t have spammy and sensationalizing language.

Following these broad guidelines will allow you to send high-quality emails for your campaign. These guidelines will also ensure that your bounce rate stays at a minimum. 

You deserve to give your business a fighting chance in this day and age. Avoiding pitfalls like high email bounce rates ensures your business reaches the people needing it the most. 

Frequently Asked Questions (FAQs)

What is an email bounce?

An email bounce occurs when your email fails to reach its recipient and is returned by the recipient’s mail server. You receive an automated Non-Delivery Report (NDR) with an SMTP error code explaining why delivery failed, such as an invalid address or full inbox.

What causes emails to bounce back?

Emails bounce due to permanent issues like invalid addresses, non-existent domains, or recipient blocks (hard bounces), or temporary issues like full inboxes, server downtime, or large attachments (soft bounces). SMTP error codes identify the specific cause of each bounce.

What is the difference between hard bounce and soft bounce?

A hard bounce is a permanent delivery failure caused by invalid addresses, blocked senders, or non-existent domains (5xx SMTP codes). A soft bounce is temporary, caused by full inboxes, server issues, or size limits (4xx codes), and may succeed if retried.

What are SMTP error codes?

SMTP error codes are 3-digit numbers returned by email servers explaining why delivery failed. Codes starting with 4 indicate soft bounces (temporary), while codes starting with 5 indicate hard bounces (permanent). Example: 550 means mailbox doesn’t exist.

How do I know if my email bounced?

When your email bounces, you receive an automated Non-Delivery Report (NDR) or bounce-back message in your inbox. The NDR includes the recipient’s address, an SMTP error code, and a description explaining why delivery failed.

Can bounced emails be delivered later?

Soft bounces (4xx codes) can often be delivered later if the temporary issue resolves, such as the recipient freeing up inbox space. Hard bounces (5xx codes) are permanent failures and cannot be delivered; remove these addresses immediately.

What is an NDR message?

An NDR (Non-Delivery Report) is an automated email sent by a mail server when your email cannot be delivered. It includes the recipient’s email address, SMTP error code, reason for failure, and sometimes a copy of your original message.

Why did my email get rejected?

Emails are rejected for various reasons: invalid or blocked email addresses, full inboxes, spam content, authentication failures (missing SPF/DKIM), blocklisted IPs, or policy violations. Check the SMTP error code in your bounce-back message to diagnose the specific issue.

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