What is a SMTP Banner?

About

A SMTP banner is the protocol banner of SMTP (ie the response that a client will get when it connects to a SMTP server)

Example

With Telnet, we connect to the local smtp server submission port

nc localhost 25

• We receive an SMTP greeting where 220 is a reply code that means all is good (an equivalent to the 200 HTTP status code)

220 server01.bytle.net ESMTP Postfix

• The banner text follows the status code

server01.bytle.net ESMTP Postfix

• where:
  • server01.bytle.net is the domain/hostname
  • ESMTP indicates that this server talks:
    • the SMTP protocol
    • where the E means Extension (such as size, chunking, …)
  • Postfix is the name of the SMTP application that:
    • has implemented the SMTP protocol
    • replies.

Postfix Configuration

• postconf.5.html it default to $myhostname ESMTP $mail_name