2xx reply
An SMTP reply starting with 2, meaning the requested step succeeded, such as 220 (ready), 250 (accepted) or 221 (closing).
What it means
RFC 5321 calls these positive completion replies. The common ones are 220, the greeting; 250, which accepts a command such as MAIL FROM, RCPT TO or the end of the message data; 221, the answer to QUIT; and 235 for a successful SMTP AUTH login. 251 says the user is not local but the server will forward the message, and 252 that the server cannot verify an address but will accept the message and attempt delivery.
The 250 after the message data is the moment responsibility moves to the receiving server. It means the message was accepted, not that it reached the inbox: the receiver may still file it as spam or quarantine it, and RFC 5321 acknowledges that some receivers drop mail they judge seriously fraudulent.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes