Mail Domain Check

Bounce diagnostics

SMTP error decoder

Paste one server response. The decoder extracts the status codes without sending the pasted text anywhere.

Local decode

INPUT

Server response

The pasted text stays in this browser. Page analytics record only an anonymous page view, never this field.

RESULT

Decoded status

No status decoded

SMTP replies use three digits such as 451 or 550. Enhanced status codes use class, subject, and detail values such as 4.7.23 or 5.7.26.

Code model

Read the class before the wording

4xxTemporary SMTP failure

Keep normal queue backoff. Repeating faster does not repair the named condition.

5xxPermanent SMTP failure

Do not repeat the exact send unchanged. Correct the condition first.

X.subject.detailEnhanced status

The second field identifies a broad area such as mailbox, routing, protocol, content, or policy.

Primary references: Google Gmail SMTP errors, RFC 3463 enhanced status codes, and RFC 5321 reply classes.