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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]
The [gsmtp] tag confirms the rejection came from Gmail's servers. Google Workspace bounces use [gcdp] instead.
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
- DKIM signing with a 1024-bit key or larger on your sending domain
- DMARC policy of at least
p=noneon the From domain - Spam complaint rate below 0.3% (target below 0.1%)
- 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. It simply tells you the bounce came from Google's own SMTP servers rather than a third-party filter sitting in front of them. If you see [gcdp] instead, the rejection came from a Google Workspace (business) account rather than a consumer Gmail inbox.
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?
The 4.7.28 error means Gmail users are hitting "Report Spam" on your messages at a rate that has triggered Google's filters. Log into Postmaster Tools to see your exact complaint rate. The fix is usually a combination of better list hygiene (removing unengaged contacts) and making your unsubscribe link obvious. The easier it is to leave, the less likely people are to reach for the spam button.
What is IP warmup and why does Google require it?
When you send from a brand-new IP address, Google has never seen it before, so it is understandably cautious. The 4.7.29 "new sender hold" is Google's way of saying "prove yourself first." The standard approach is to start small (a few hundred messages to your most engaged subscribers) and gradually double your volume each week. Most senders work through the warmup period in four to six weeks.
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.
Other Provider Directories
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