DATA command

What it means

Once recipients are accepted, the client sends DATA. The server answers 354 to invite the content, or refuses, for example with 503 or 554 when no recipient was accepted. The client then streams the header and body and ends with CRLF.CRLF, a line containing only a dot; a line of text that begins with a dot is sent with an extra dot (dot-stuffing) so it cannot end the message early.

The reply after the final dot is the one that counts. A 250 means the server has taken responsibility for delivering the message, so any later failure has to come back as a bounce message. Content and authentication checks usually run at this point, which is why many policy rejections arrive after DATA rather than after RCPT TO. RFC 5321 tells clients to wait at least 10 minutes for that reply; the CHUNKING extension replaces DATA with BDAT.