How to Fix an Email That Bounced Back, Code by Code

You'll hear that a bounce means the other person's inbox is full. Sometimes it is. Most of the time the bounce tells you exactly what broke, in a short code. Under RFC 3463, the internet standard for those codes, the first digit says whether to try again and the second says where the problem lives.¹

So stop guessing and read the code. It takes about a minute, and it tells you whether the fix is on your side, their side, or nobody's.

Find the Code in the Bounce

Open the bounce message and scroll past the friendly part. Near the bottom you'll find a line with a three digit number and then three numbers split by dots, like 550 5.1.1. That second part is the one to read.

If the line ends with gsmtp, the bounce came from Gmail.² Outlook and Microsoft 365 bounces put the code in a section for the sender's admin, along with the server that refused the message.³

The Bounce Codes Small Businesses Hit Most

These come from Google's Gmail SMTP error list² and Microsoft's Exchange Online bounce list.³

Look at 5.2.2 twice. On Gmail it means the other person's mailbox is full. On Microsoft it means you sent too much, and Microsoft says that can be a sign your account was taken over and is sending spam.³ Same code, opposite problem. If Microsoft sends it, change that mailbox's password before you do anything else.

What We Found on 1,285 Business Domains

Most of the fixable bounces above come back to three DNS records. SPF lists which servers may send mail for your domain. DKIM signs each message. DMARC tells the receiving server what to do when those checks fail. Our SPF, DKIM and DMARC guide explains each one.

I wanted to know how often real small businesses get these wrong. So on October 28, 2026 we took 2,000 independent auto repair shop domains at random from the 22,183 we track and looked up their public mail records. 1,285 had a working mail record, so they can send and receive email.

• 312 had no SPF record at all. That's 24 percent. Gmail requires every sender to have SPF or DKIM, and mail that has neither can be rejected with 5.7.26.⁴
• 13 had two SPF records. That sounds safer. It isn't. The SPF standard says two records produce a permanent error, so SPF fails for every message.⁵ This usually happens when a new email provider adds its own record instead of editing the old one.
• 839 had no DMARC record. That's 65 percent. 329 more had DMARC set to none, which watches but doesn't block. Only 107 told receivers to quarantine or reject mail that fails.

Paying for Google Workspace or Microsoft 365 didn't fix it either. 510 of the domains used one of the two. 91 of those had no SPF record, and 333 had no DMARC record. The provider runs the mailbox. The DNS records are still the owner's job.

We couldn't count DKIM. Each provider hides its DKIM key under a name we'd have to guess, so a public lookup can't see it. Your mail provider's admin screen can.

Fix It in This Order

1. Read the code. A 5.1 code means a bad address. Fix the spelling and send again. A 5.2 code on Gmail means their mailbox is full, so call or text them instead.
2. If it starts with 4, wait. Your server retries on its own. For a 4.7.28 from Gmail, Google says to pause for at least 10 minutes and then send from a single connection.⁴
3. If it's a 5.7 code, check your domain. Run it through our free email security checker. It shows your SPF and DMARC records in plain words.
4. Keep exactly one SPF record. List every service that sends as you in that one record, like your mailbox provider and your invoicing software. The standard caps SPF at 10 DNS lookups. Go over and SPF fails.⁵
5. Turn on DKIM in your mail provider's admin screen. It gives you a record to add at your DNS host.
6. Add DMARC at p=none to start. Google accepts none even for its bulk sender rule.⁴ Move to quarantine once your reports look clean.
7. If it's 5.7.25 or a blocked server, the problem is your email host's server, not your records. Send the bounce to your host.

Google's rules split senders in two. Everyone needs SPF or DKIM, valid reverse DNS, and a secure connection. Anyone sending more than 5,000 messages a day to Gmail also needs SPF, DKIM and DMARC.⁴ A shop sending invoices isn't a bulk sender. But the everyone rules still apply, and those are the ones the 312 domains in our check missed.

When the Bounce Isn't Yours

If customers say their mail to you bounces back, the problem sits on your side of the wire, not theirs. Start with our guide to not receiving emails, which works through MX records and full mailboxes.

And if you get bounces for mail you never sent, someone may be faking your address. A DMARC record set to quarantine or reject is what tells other servers to stop trusting those fakes.

Get Help With Business Email

T3CHS sets up business email at your own domain, adds the records for you and checks that mail arrives. Call or text (949) 65-T3CHS, or send us the bounce and we'll tell you what it means.

Before You Call

What are the common reasons why an email bounces back?

A wrong or closed address, a full mailbox, a message that's too big, or a domain that fails SPF, DKIM or DMARC checks. The code in the bounce tells you which one. A code starting with 5.1 is the address, and 5.7 is usually your domain's records.

Should I delete bounced emails?

Keep the bounce until you've read the code and fixed the cause. It holds the exact error and the server that refused your message. Remove an address that hard bounced with 5.1.1, since sending to it again won't work.

Can you provide an example of an email bounceback message?

A common one from Gmail reads 550 5.1.1 with a note that the account you tried to reach does not exist. Another is 550 5.7.26, which means your message failed SPF and DKIM or was blocked by DMARC.

Why do my emails bounce back from Gmail but not Outlook?

Gmail checks sender records strictly. Every sender needs SPF or DKIM, and mail with neither can be rejected with 5.7.26. Check that your domain has one SPF record and DKIM turned on.

Does a bounced email mean I've been hacked?

Not usually. But a Microsoft 5.2.2 bounce means you went over your sending limit, and Microsoft says that can mean the account is being used to send spam. Change the password and check your sent folder.

Cited Pages

1. RFC 3463: Enhanced Mail System Status Codesrfc-editor.orgOpened October 2026
2. Google Workspace Help: Gmail SMTP Errors and Codesknowledge.workspace.google.comOpened October 2026
3. Microsoft Learn: Email Nondelivery Reports and SMTP Errors in Exchange Onlinelearn.microsoft.comOpened October 2026
4. Google: Email Sender Guidelinessupport.google.comOpened October 2026
5. RFC 7208: Sender Policy Framework, Sections 4.5 and 4.6.4rfc-editor.orgOpened October 2026

Last reviewed November 1, 2026.