What is a bounce-back email?
A bounce-back email is the automatic notice your email provider sends you when a message you sent could not be delivered. Microsoft's guide to nondelivery reports calls it a delivery status notification (DSN), and says the notice is also known as a bounce message or bounce-back message. The most common kind is a nondelivery report (NDR), which carries an error code, a suggested fix and technical details for administrators.
In Gmail, Google's help page on bounced emails says to look in your Inbox and your Spam folder for a message from Mail Delivery Subsystem ([email protected]). Its subject line is often Delivery Status Notification (Failure). The error message inside tells you why the email didn't arrive.
People also say "bounce back" for an out-of-office auto-reply. That is a different thing: an auto-reply means your email was delivered and the other mailbox answered it. If that is what you need, skip to the auto-reply section below.
How do you read a bounce-back message?
Find the two codes in the error text: a three-digit reply code such as 550, then an enhanced status code such as 5.1.1. The first digit of each tells you whether to retry. Here is a sample, built from Gmail's documented wording for a missing address (the recipient is invented):
The enhanced code has three parts, defined in RFC 3463 (January 2003):
- Class, the first number. 2 is success. 4 is a persistent transient failure: "sending in the future may be successful". 5 is a permanent failure, one "not likely to be resolved by resending the message in the current form".
- Subject, the middle number. 1 is the address, 2 the mailbox, 3 the receiving mail system, 4 network and routing, 5 the delivery protocol, 6 message content, and 7 security or policy.
- Detail, the last number. For example X.1.1 is a bad destination mailbox address, X.2.2 a full mailbox and X.7.1 delivery not authorised.
The three-digit reply code follows the same split. RFC 5321, the SMTP standard, says 4yz replies are temporary and the client should try again, while for 5yz replies the client should not repeat the exact request. Gmail adds gsmtp to its errors, and gcdp when the rejection comes from a custom rule a Google Workspace administrator created, according to its SMTP error reference. A gcdp tag means an administrator's custom rule blocked the message, not a broken address.
What do common bounce codes mean, and what should you do?
Here are 14 codes, in eleven rows, from Google's and Microsoft's lists, with what each page says they mean (checked 4 November 2026) and what to do next. The "type" column follows the first digit of the code, except the two rows marked Your account, which are about your own sending.
| Code | What the provider says | Type | What to do |
|---|---|---|---|
| 550 5.1.1 (Gmail), 5.1.1 or 5.1.10 (Microsoft) | The account doesn't exist; bad destination mailbox; recipient not found | Permanent | Check for typos. If it is spelled right, remove the address |
| 553 5.1.2 (Gmail) | The recipient domain couldn't be found | Permanent | Check the part after the @, and any stray punctuation after it |
| 550 5.2.1 (Gmail) | The account is inactive | Permanent | Remove it and find a current address. Gmail also uses 5.2.1 when the recipient is receiving mail too fast, so read the text first |
| 452 4.2.2 (Gmail) | The recipient's inbox is out of storage space | Temporary | Send again later; the recipient has to clear space |
| 552 5.2.2 (Gmail) | Out of storage space and inactive | Permanent | Reach the person another way |
| 421 4.7.28 (Gmail) | An unusual rate of email, so it has been temporarily rate limited | Temporary | Send less, spread across the day |
| 550 5.7.26 (Gmail), 5.7.23 or 5.7.509 (Microsoft) | The sender is unauthenticated, failed SPF, or failed DMARC with a reject policy | Permanent | Fix SPF, DKIM and DMARC before sending again |
| 550 5.7.1 (Gmail and Microsoft) | A policy prohibits the email, the message looks unsolicited, or you aren't allowed to send to that group | Permanent | Don't resend the same message; ask the recipient or their admin |
| 4.4.7 (Microsoft) | The message expired in the queue before it could be delivered | Temporary, now final | Check the address and whether the receiving server accepts mail |
| 550 5.4.5 (Gmail) | Daily user sending limit exceeded | Your account | Wait; on a personal Gmail account Google says you can send again within 24 hours |
| 5.1.8 (Microsoft) | The account was blocked for sending too much spam, typically because it was compromised | Your account | Secure the account, then follow Microsoft's fix page for 5.1.8 |
Four rows point at your own sending rather than the address: the rate-limit row (4.7.28), the authentication row and the two marked Your account. The 5.7.26, 5.7.23 and 5.7.509 codes mean authentication failed, which affects every recipient who checks it. Our fix list for cold emails landing in spam walks through checking SPF, DKIM and DMARC in Gmail's Show original. Google's bounce help page says the sending-limit error on a personal Gmail account comes from sending more than 500 emails in a day or one email to more than 500 recipients; Google Workspace accounts have different limits.
What happens if an email bounces back?
The message is not delivered and the recipient never sees it. With a temporary error, RFC 5321 requires the sending server to queue the message and retry it first. RFC 5321 says the retry interval should generally be at least 30 minutes and the give-up time generally needs to be at least 4 to 5 days. So a bounce for a temporary problem can arrive days after you sent the message. Google's advice for a temporary problem with the recipient's inbox is to send the message again later.
For a small team sending outreach, sort each campaign's bounces by code before you delete anything. A worked example with sample numbers:
| Bounce code | Count (of 400 sent) | Share of sends | Action |
|---|---|---|---|
| 550 5.1.1 | 9 | 2.25% | Remove all nine addresses |
| 550 5.7.26 | 3 | 0.75% | Fix authentication before the next send |
| 452 4.2.2 | 2 | 0.5% | Retry once in a few days |
| Total | 14 | 3.5% |
The arithmetic: 9 ÷ 400 = 2.25%, 3 ÷ 400 = 0.75%, 2 ÷ 400 = 0.5%, and 14 ÷ 400 = 3.5% in all. The headline 3.5% hides the more urgent problem: the three authentication bounces point to your own domain setup, and Gmail's 5.7.26 text says it requires all senders to authenticate with SPF or DKIM. Track authentication bounces apart from bad addresses, the same way our bounce rate definition separates hard and soft bounces in its example.
When many 5.1.1 bounces come from one list, a likely cause is old data: people change jobs and their work addresses stop working. Our piece on why exported B2B contact data goes stale covers how to re-check a list before you send.
One more case: a bounce for an email you never sent. Microsoft's NDR guide explains that a spammer can forge your address as the sender, so the bounce comes back to you. If your Sent folder doesn't have the message, check your account security and your domain's SPF and DMARC records, and don't reply to the bounce.
How do you send a bounce-back email or auto-reply?
It depends which "bounce back" you mean. The Gmail and Outlook help pages we checked cover automatic replies and, in Gmail, blocking; none of them describes a way for a user to send a fake delivery failure. An Exchange Online administrator can reject mail with a real bounce.
An out-of-office auto-reply in Gmail (computer). From Google's vacation responder help page: open Gmail, click Settings, then See all settings. On the General tab, go to Vacation responder and select Vacation responder on. Fill in the date range, subject and message, optionally tick the box to reply to your contacts only, and click Save Changes. Google says the reply usually goes to each person only the first time they message you, and again if they write after four days while it is still on.
An automatic reply in new Outlook for Windows. From Microsoft's automatic replies help page: on the View tab, select View settings, then Accounts, then Automatic replies. Turn the toggle on, set a time period, and select Save. With a work or school account, Microsoft warns that replying outside your organisation sends the reply to newsletters and possibly junk too, and recommends replying only to contacts. The same page says automatic replies aren't supported for Gmail, Yahoo or other POP or IMAP accounts in new Outlook.
Returning unwanted mail to its sender. In Gmail, blocking a sender sends their future emails to Spam, according to Google's blocking help page; it doesn't bounce them. In Exchange Online, an admin can create a mail flow rule with the action Reject the message with the explanation. Microsoft's mail flow rule actions page says this returns the message to the sender in an NDR with your text as the reason, and the recipient doesn't receive it.
Bounce-back email templates you can copy
We wrote these for small B2B teams; they have no measured reply rates behind them. Replace the square brackets, and keep the sample names out of real sends.
Each follow-up line names a date and a deliverable, so the other person knows exactly what to expect. For template 2, if you need the person's new work address, our guide on guessing a work email pattern and then verifying it shows how.
This week: search your mailbox for Delivery Status Notification or mailer-daemon, sort what you find by code using the table above, and after checking for typos, remove every address that returned 5.1.1 or 5.1.10. A 5.1.8 is about your own account, not the address.
Frequently asked questions
How do I see bounce back emails?
In Gmail, check both your Inbox and your Spam folder for a message from Mail Delivery Subsystem, usually titled Delivery Status Notification (Failure). Microsoft 365 sends a nondelivery report to you with an error code and suggested fix. Searching your mailbox for those phrases finds older bounces too, so you can clean a list in one sitting.
Can you bounce an email back to the sender?
Not with the Gmail and Outlook features we checked. Gmail's block option sends a sender's future emails to Spam rather than returning them. An Exchange Online administrator can set a mail flow rule that rejects matching messages and sends the sender a nondelivery report with a custom reason, so the recipient never receives them.
Does a bounced email mean the person blocked me?
Not necessarily. Gmail's help page says blocking sends future emails from that sender to Spam, not back as a bounce. Many bounce codes are about the address itself, such as a 5.1.1 code for an account that doesn't exist. A 5.7.1 code is the one to read closely: it means a policy or spam filter refused the message, and Gmail's text says the policy can sit with the recipient's domain or yours.
Will a soft-bounced email be delivered later?
Often the sending server already tried. The SMTP standard says retries should generally be at least 30 minutes apart, with a give-up time of at least four to five days, so the notice may arrive after those attempts ended. For a temporary problem with the recipient's inbox, Google's advice is to send the message again later.
Is an out-of-office reply the same as a bounce?
No. An out-of-office reply means your email reached the mailbox and the person set an automatic answer while away. A bounce means it never arrived. Keep the address after an auto-reply, note the return date it gives, and follow up after that date instead of removing the contact from your list.
Sources
- Gmail Help: Fix bounced or rejected emails (checked 4 Nov 2026)
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 4 Nov 2026)
- Microsoft Learn: Email nondelivery reports and SMTP errors in Exchange Online (checked 4 Nov 2026)
- Microsoft Learn: Mail flow rule actions in Exchange Online (checked 4 Nov 2026)
- IETF RFC 3463: Enhanced Mail System Status Codes (January 2003)
- IETF RFC 5321: Simple Mail Transfer Protocol (October 2008)
- Gmail Help: Send an automatic reply when you're out of office (checked 4 Nov 2026)
- Microsoft Support: How to set up out of office automatic replies in Outlook (checked 4 Nov 2026)
- Gmail Help: Block an email address in Gmail (checked 4 Nov 2026)