A collection of raw SMTP responses spotted in the wild

Find a specific SMTP code or email provider

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SMTP codes

4.X.X Persistent transient failure

The mail server encountered a temporary failure. If the command is repeated without any change, it might be completed. Mail servers can use temporary failures like this to keep untrusted senders at bay.

5.X.X permanent error

The mail server has encounted a permanent error. These errors will result in the SMTP connection being dropped. Re-sending will usually produce the same result.

Email providers

View SMTP responses for a specific email service provider.

Spam filters

View SMTP responses for a specific spam filter service.

SMTP Field Manual

The SMTP field manual is a collection of raw SMTP server responses from major email service providers and spam filter services. The data is open source so anyone make contributions. Even you! See something interesting from an email service provider or simply have a suggestion? Let us know.

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