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↩️ Google / Gmail SMTP Error Code Directory
Google stamps its bounce messages with proprietary identifiers like gsmtp alongside standard SMTP codes. This directory covers every Gmail and Google Workspace rejection you are likely to run into, with a plain-English explanation and a fix for each one.
How to Identify a Gmail Bounce
Gmail NDRs arrive from [email protected]. The Diagnostic-Code line contains the SMTP code, enhanced code, a human-readable reason, and ends with a [gsmtp] or [gcdp] tag.
Diagnostic-Code: smtp; 550 5.7.26 This message does not pass authentication checks (SPF and DKIM both do not pass). To fix this issue, review your SPF and DKIM configuration for example.com. [gsmtp]
Google appends [gsmtp] to every error it issues. It appends [gcdp] when the rejection came from a custom rule a Google Workspace administrator wrote, so that tag means a person configured this and no authentication fix will clear it.
Gmail Error Codes
13 codes with Gmail-specific behaviour. Critical (Tier 1) codes appear first.
Google Bounce Identifier Glossary
Remote-MTA in your bounce contains this, the rejection happened at Gmail's receiving gateway.Gmail 2024 Bulk Sender Requirements
Since February 2024, Google requires all senders of more than 5,000 messages per day to Gmail addresses to meet these standards. Failure results in rejections and deferrals:
- Valid SPF record authorising your sending IPs, and DKIM signing on your sending domain (either one alone satisfies the rules for senders under the threshold)
- A DMARC record on the From domain. The policy may be
p=none; it just has to exist - The From domain aligned with either the SPF domain or the DKIM domain
- Valid forward and reverse DNS (PTR) for your sending IPs, and TLS on outbound connections
- Postmaster Tools spam rate below 0.3% (target below 0.1%), and this one applies to every sender, not just bulk
- One-click unsubscribe header for marketing mail
Frequently Asked Questions
Common questions about Gmail SMTP errors and delivery failures.
What does [gsmtp] mean in a bounce message?
The [gsmtp] tag is Google's stamp on the end of its rejection messages. It is not an error code. Google documents it as being added to all errors it issues, so it tells you Google generated the diagnostic rather than a third-party filter in front of it, but not which Google service. [gcdp] is the informative one: Google Custom Domain Policies, added when the rejection came from a custom rule a Google Workspace administrator wrote. That one is resolved by the receiving organisation, not by fixing your authentication.
How do I check my sending reputation with Google?
Google offers Postmaster Tools for free. Once you verify your sending domain, it shows you your domain reputation, IP reputation, spam complaint rates, and a breakdown of any delivery errors. It's one of the most useful free tools available to email senders and well worth setting up.
Why is Google blocking my mail with 4.7.28?
Because Gmail has seen an unusual volume of mail tied to one of your identifiers and has rate limited it. Every 4.7.28 text Google publishes describes an unusual rate of email, attributed to your IP address, your netblock, your SPF domain, your DKIM domain, a URL in your message body, or a quota on repeated Message-IDs. Recipient complaints feed into Gmail classifying that volume as unsolicited, but the code is not a complaint-rate code. Read the sentence to see which identifier was limited, then check your spam rate in Postmaster Tools and keep it under 0.3%.
Does 4.7.29 mean I need to warm up my IP?
No. Google documents 4.7.29 as "Your email has been rate limited because you're not using a TLS connection", and requires bulk senders to use TLS/SSL for SMTP. It is a configuration fix on your side, not a reputation decision, and it clears the moment the connection is encrypted. Check that your MTA offers STARTTLS on outbound mail, that your certificate is valid, and that nothing in the path is stripping the STARTTLS advertisement. (Worth knowing: IANA registers X.7.29 as "ARC validation failure", so this code does not carry its registered meaning at Gmail.)
Why does SPF pass but my email still gets rejected with 5.7.26?
DMARC has a concept called alignment that trips a lot of people up. SPF passing is not enough on its own. The domain that SPF authenticates must also match the domain in your From address. If you use a third-party sending service, SPF passes for their domain, not yours. The only reliable fix is to set up DKIM signed with your own domain, which gives you the alignment DMARC needs.
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