Exchange 2003 sp2.
The receiving mail server sees the *internal* fqdn in the DATA and BDAT command, rather than the *external* fqdn as defined in the SMTP virtual server-->delivery-->advanced tab.
I've checked on 2 different Exchange servers and see the same behavior.
Exch server #1 (this is in the recipient's smtp log files)
EHLO - +mail.external-domain-name.com 250
BDAT - +<7D435A70B4195B4AA2304D7C34BB5F3202167B@server03.internal-domain.local>
QUIT - mail.external-domain-name.com
Exch server #2 (in this case, the mail goes through a 3rd party filtering service before reaching the destination Exchange server)
EHLO - +barracuda2.3rd-party-provider.net
DATA - +<0FF89AA0A405354B84D76464033F84D186AA99@myserver.different-internal-fqdn.local>
QUIT - barracuda2.3rd-party-provider.net