SMTP banner
The first line a mail server sends when a client connects: a 220 reply naming the server, sent before any command is exchanged.
What it means
In SMTP the server speaks first. When it accepts a connection it sends a 220 reply that starts with its own name, for example 220 mx.example.net ESMTP, and may add its software and version. A server that will not take mail from this client can greet with 554 instead of 220; it must then wait for the client to send QUIT and should answer anything else with 503.
The client has to wait for the greeting before sending commands. RFC 5321 sets a minimum of five minutes for that wait, because busy servers sometimes hold the greeting back until they have capacity. The name in the banner is also how a sender can tell from its logs which host refused a message.
Example
220 mx.example.net ESMTP readyRelated terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 3.1: Session Initiationrfc-editor.org
- RFC 5321, section 4.5.3.2.1: Initial 220 Message: 5 Minutesrfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes