[TYPO3-debian] Mail Server not on localhost (here: another machine in LAN)

[M. Buecher]

Hello everybody,

a friend created a homepage for me with Typo3 CMS, we moved it (tar.gz) to my web server machine, got Typo3 running and also serving the contents. So far, so good.
Where we got stuck is the mail connection to my mail server which runs on a different machine in the same subnet.
Unfortunately he has no experience with this situation in conjunction with Typo3.

Typo3 v6.2.10 running on Ubuntu 14.04 LTS; Exim4 replaced with nullmailer 1:1.11-2.1 as no SMTP server (MDA/MTA) is needed just a client (MUA). As far as I see our Typo3 Installation is using `/usr/sbin/sendmail -bs`.
Mail server is running Exchange 2013 on a different machine in the same subnet.
SMTP connection has been succesfully tested with telnet on port 25.

What are the typical setups or best practices for this situation?

If any further information is needed, then just ask.

Any help is greatly appreciated
Maddes