Mail Domain Check

Bounce diagnostics

SMTP error decoder

Paste one server response or Java TLS exception. The decoder separates certificate identity failures, retryable 421 and other 4xx replies from permanent 5xx outcomes without sending the text anywhere.

Local decode

INPUT

Server response

Known failure

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

RESULT

Decoded status

No status decoded

Paste a three-digit SMTP reply, enhanced status code, or supported TLS certificate identity exception.

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.

TLS identityHandshake before SMTP

Keep the original DNS hostname for SNI and certificate verification even when the TCP connection is pinned to a validated IP.

Primary references: Oracle JSSE SNI and endpoint identification, Google Gmail SMTP errors, Microsoft 554 5.2.252 SendAsDenied guidance, Microsoft S3150 guidance, RFC 3463 enhanced status codes, and RFC 5321 reply classes.