3-digit reply code

SMTP error 354

Start mail input.

Updated Jul 1, 2026

The short answer

Start mail input. Common cause: The response to DATA — the server is ready to receive the message body.

Quick fix

A normal mid-conversation step, not an error.

How to fix it

A normal mid-conversation step, not an error.

All SMTP error codes
Ready when you are

Most rejections come back to sender reputation.

Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.

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