Question : SMTP Sessions and Domains

Hello,

How does a mail server correctly determine the domain of a connecting server? For example -
(private addresses used for example) 172.16.1.1 establishes an SMTP session with 10.0.1.1 and announces itself as mail.domainb.com in SMTP banner.
does 10.0.1.1 accept domainb.com as the domain for the conneting mail server or does it require further verification such as performing a DNS lookup on mail.domainb.com and if so should this be the same as the connecting server's IP and if not is a mail server justified in aborting the session?

Answer : SMTP Sessions and Domains

well, the dns lookup is not required by rfc 2821, but most well-configured mail servers will do some kind of dns lookup to verify the message origin...

as a matter of fact, most of the time, if you want to validate a message's origin, you won't use the id provided in the ehlo command, you'll do a dns query looking for a SPF record... it will tell you whether a server is authorized to send mail from a particular domain...

you should probably read this:
http://www.ietf.org/rfc/rfc2821.txt
http://en.wikipedia.org/wiki/Sender_Policy_Framework