Deliverability: guides by provider and by error
Two entry points for the same problem. You know who hosts your mail: follow your provider's procedure. You have a refusal message in front of you: look up its code. Each page cites the official documentation it is drawn from.
Not sure of the cause? Start with the diagnostic
The free diagnostic reads the SPF, the DMARC and the DKIM signature of your domain, with no account. It tells you in a few seconds whether authentication is the problem, before you read a guide.
Test my domainBy provider: SPF, DKIM and DMARC
The menus, the records to create, the announced delays and the pitfalls specific to each provider, taken from its documentation.
By error: understanding a refusal message
An SMTP code is read in two steps. The first digit says whether the refusal is temporary (4) or permanent (5). The following numbers, like 5.7.1, specify the category. But it is the text that follows which tells the causes apart: each page explains how.
| Code | What it means | Guide |
|---|---|---|
| 4xx (421, 451, 450...) | Temporary refusal: the server will retry. Keep an eye on it if it repeats. | Read |
| 550 5.1.1 | The recipient address does not exist. | Read |
| 550 5.7.1 | Policy refusal: reputation, content, IP or a recipient rule. | Read |
| 550 5.7.26 | Unauthenticated message: SPF, DKIM or DMARC. | Read |
| 554 | Generic permanent refusal: session, malformed message, domain, policy. | Read |
- Error 550 5.7.1: real causes and how to fix themPolicy refusal: reputation, spam, unauthorized IP, recipient rule.
- Error 550 5.7.26 Gmail: SPF, DKIM and DMARC to fixGmail rejects a message without valid SPF or DKIM, or rejected by the DMARC policy.
- Error 550 5.1.1: address does not exist, what to do?The recipient address does not exist: typo, closed mailbox or outdated list.
- Errors 421 and 451: deferral, throttling and retry delaysTemporary refusals: deferral, rate limiting, reputation being built. When to retry and when to slow down.
- Error 554: what permanent SMTP refusals hideVery general permanent refusal: session refused, malformed message, loop, invalid domain, policy.
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