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.
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.
13 codes with Gmail-specific behaviour. Critical (Tier 1) codes appear first.
Remote-MTA in your bounce contains this, the rejection happened at Gmail's receiving gateway.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:
p=none on the From domainCommon questions about Gmail SMTP errors and delivery failures.
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.
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.
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.
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.
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.
Paste your full NDR email or SMTP error line for an instant plain-English diagnosis.
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