Gmail Bounce Codes: How to Find Out Why Gmail Rejected Your Email

To find out why Gmail bounced your email, read the SMTP reply in the bounce: a three-digit code, an enhanced code such as 5.1.1, and a line of text. Match the enhanced code to Google's Gmail SMTP error reference. Then fix the group it falls in. Recipient codes mean the address is dead. Authentication codes mean SPF, DKIM or DMARC failed. Reputation codes mean Gmail distrusts your traffic. Format codes mean the message itself broke a rule. Limit codes mean your own account sent too much.

This guide shows where to find the code, how to read it, and what to do with each group.

Where to find the bounce reason

The reason is in one of three places, depending on how you send.

From Gmail or Google Workspace, sending by hand. The bounce arrives as a message from "Mail Delivery Subsystem", often with the subject "Delivery Status Notification (Failure)". Google's help page on fixing bounced or rejected emails describes it. Open it and look for the technical details near the bottom.

From a sequencer or ESP. Open the bounce record or event log for the contact. Most tools show the remote server's reply. If yours only says "bounced", export the raw event or check the bounce message in the sending inbox.

As a Google Workspace admin. Email Log Search lets admins find and review messages sent and received in the last 30 days. It is useful when users report bounces they deleted.

For volume senders, Postmaster Tools adds dashboards for spam rate, reputation, authentication and delivery errors. It only covers mail to gmail.com and googlemail.com addresses, and Google says data can be missing on low-volume days. Our Postmaster Tools guide explains how to read it.

Is it really Gmail?

Gmail is not only gmail.com. Companies on Google Workspace receive mail through Google too. Google tells Workspace admins to set the MX record to smtp.google.com, and older setups use values starting with aspmx, per Google's MX setup page. If a bounce came from a Google host, the codes below apply, whether the recipient was @gmail.com or @somecompany.com. Google's replies usually end with - gsmtp.

How to read a Gmail bounce line

A typical line looks like this:

550 5.7.26 This email has been blocked because the sender is unauthenticated. ... - gsmtp

• 550 is the basic SMTP code. A first digit of 5 means permanent; 4 means temporary.
• 5.7.26 is the enhanced status code. RFC 3463 defines the structure: the middle number is the subject. 1 is addressing, 2 is the mailbox, 3 is the mail system, 4 is network and routing, 5 is the mail delivery protocol, 6 is message content, 7 is security or policy.
• The text is Google's explanation, often with a link to a help article.

So 5.1.x means the address, 5.2.x the mailbox, and 5.7.x a security or policy decision about you or the message.

Gmail bounce codes, grouped by cause

All texts below are quoted from Google's SMTP error reference, checked on 13 October 2026. Some codes have several variants, so match on the text as well as the number.

Group 1: the recipient

Code	Google's text	What to do
550 5.1.1	"The email account that you tried to reach does not exist."	Check for typos. Suppress if correct
550 5.2.1	"The email account that you tried to reach is inactive."	Suppress
552 5.2.2	"The recipient's inbox is out of storage space and inactive."	Suppress
452 4.2.2	"The recipient's inbox is out of storage space."	Temporary. Retry later, suppress if it repeats
450 4.2.1	"The user you are trying to contact is receiving email too quickly."	Temporary. Retry later

This is the only group that list verification prevents. A verifier that checks the mailbox over SMTP, without sending, finds most 5.1.1 addresses before you send. Our guide on checking whether a Gmail address exists explains what Gmail does and does not reveal to a verifier.

Group 2: authentication

Code	Google's text (start)	What to do
550 5.7.26	"This email has been blocked because the sender is unauthenticated." Other variants cite the domain's DMARC policy or an SPF hard fail	Set up SPF and DKIM, then DMARC
421 4.7.26	"This email has been rate limited because it is unauthenticated."	Same fix. Temporary for now
550 5.7.27 / 421 4.7.27	Blocked or rate limited because the message "didn't pass SPF authentication"	Fix the SPF record for the envelope sender domain
550 5.7.30 / 421 4.7.30	Blocked or rate limited because DKIM "didn't pass"	Fix DKIM signing and the published key
421 4.7.32	Rate limited because the From: header "isn't aligned"	Align the From domain with the SPF or DKIM domain
550 5.7.40 / 421 4.7.40	The sending domain "doesn't have a DMARC record", or the record has a problem	Publish a valid DMARC record

Google's email sender guidelines require SPF or DKIM for everyone. Senders of more than 5,000 messages a day to Gmail accounts also need DMARC and From alignment. Our side-by-side of the Gmail and Yahoo bulk sender requirements covers the full list.

Group 3: infrastructure

Code	Google's text (start)	What to do
550 5.7.25	"This message was blocked because the sending IP address doesn't have a PTR record"	Add reverse DNS that matches forward DNS
421 4.7.0 (PTR variant)	"The IP address sending this message does not have a PTR record..."	Same fix
550 5.7.29 / 421 4.7.29	Blocked or rate limited because the message "wasn't sent over a TLS connection"	Enable TLS on the sending server

Most sequencers and ESPs handle these for you. They usually appear with self-hosted servers, where you control the IP and its DNS records.

Group 4: reputation and rate

Code	Google's text (start)	What to do
421 4.7.28	"Gmail has detected an unusual rate of email." Variants name your IP, DKIM domain, SPF domain or a URL domain	Slow down. Spread volume
550 5.7.28	"There is an unusual rate of unsolicited email originating from your IP address."	Stop, review targeting and complaint rate
550 5.7.1 (spam variant)	"This message is likely unsolicited email. To reduce the amount of spam sent to Gmail, this message has been blocked."	Review list source, content and complaints
421 4.7.0 / 550 5.7.1 (reputation variants)	"...due to the very low reputation of the sending IP address" or "sending domain"	Rebuild reputation before scaling

These codes are about how Gmail sees your traffic. Google tells senders to keep the spam rate in Postmaster Tools below 0.3%, and ideally below 0.10%. A list full of dead or unrelated addresses can contribute to reputation problems, but the code itself is about you, not one recipient. If you are here, our sender reputation recovery plan is the next read.

Group 5: message format and size

Code	Google's text (start)	What to do
550 5.7.1 (format variants)	"Messages missing a valid Message-ID: header are not accepted", "multiple From: headers", "not RFC 5322 compliant"	Fix how your tool builds headers
554 5.6.0	"Email message is malformed. Not accepted."	Fix the message encoding
552 5.3.4	"Your message exceeded Google's message size limits."	Reduce attachments or size

Group 6: your own account's limits

Code	Google's text	What to do
550 5.4.5	"Daily user sending limit exceeded."	Wait, and lower daily volume per inbox
550 5.7.1 (relay variant)	"Daily SMTP relay sending limit exceeded for this customer."	Same, for SMTP relay

These come back when you send through Gmail or Workspace and go over a limit. Google's help page says personal Gmail accounts hit the limit at more than 500 emails a day, or one email to more than 500 recipients. Workspace limits differ. See Gmail sending limits.

Temporary system codes

421 4.3.0 "Temporary System Problem. Try again later." and 451 4.3.0 mean the problem is on Google's side or transient. Let your server retry.

A diagnosis order that saves time

When a batch of Gmail bounces arrives, sort them in this order:

1. Count 4xx versus 5xx. 4xx codes are temporary. Google's guidelines say that if messages start bouncing or being deferred, you should reduce volume until the error rate drops.
2. Pull out every 5.7.x. These are about you. Check authentication first, then reputation.
3. Pull out 5.4.5. That is your own account's limit.
4. What remains in 5.1.x and 5.2.x is the list. Suppress those addresses and check where they came from.

If a list you verified still produces 5.1.1 bounces, our guide to verified emails that still bounce walks through the usual causes, from time gaps to catch-all segments.

Worked example (illustrative)

The bounce lines below are made up to show the method. They follow Google's published texts.

A founder sends 300 cold emails from a new Workspace inbox. Next morning the sequencer shows 41 bounces.

Bounce line seen	Count	Group	Action
550 5.7.26 ... sender is unauthenticated	30	Authentication	DKIM was never turned on. Enable it, add DMARC
421 4.7.28 ... unusual rate of email	5	Reputation and rate	Cut the daily volume for the new inbox
550 5.1.1 ... does not exist	6	Recipient	Suppress, verify the rest of the list before resuming

The list was responsible for 6 of 41 bounces. Verification would have caught most of those. The other 35 would have happened with a perfect list. The right order is to fix DKIM, slow down, verify the remaining contacts, and only then resume. Our Google Workspace DKIM setup guide covers the first step.

Preventing the recipient bounces

For the one group the list controls:

• Verify addresses before the first send, and again before re-mailing an old list.
• Send only the confirmed file to the main campaign. Hold catch-all and unknown results for a smaller test.
• Suppress every 5.1.1, 5.2.1 and 5.2.2 address permanently.

You can check single addresses with the email verifier tool, or a whole CSV in one upload. SimpleVerifier's pay-as-you-go packs cost $9 for 2,500 or $19 for 10,000 verifications, with credits that never expire. Pro is $29.99 a month for unlimited email verification and API access. See pricing.

The short version

Gmail tells you why it bounced a message. Read the enhanced code and the text. 5.1.x and 5.2.x mean the address is dead or inactive: suppress it and verify the rest of the list. 5.7.x means authentication, infrastructure, reputation or format: fix your setup before you send more. 5.4.5 means your own account hit its limit. Anything starting with 4 is temporary, so slow down and let your server retry.