I have no idea how the other major MTAs handle this, but sendmail seems to issue an ident-auth-request on each connect it gets.
As i think this is rather useless, as most hosts don't answer it, and, even if they would, the answer is easily fakeable, i switched it off.
to /etc/mail/sendmail.mc disables this. Sadly this isn't documented, for confTO_IDENT it only states
The timeout waiting for a response to an IDENT query.
thanks goes to http://sial.org/howto/sendmail/tips/
addendum: 'Your Name' in the comments is right. ident is still of some use on multiuser-systems to the local admin. But in the field of Internet-mailservers it is useless.