[PEN-TEST] eMail auditing problem

["Groh, Jens" <jgroh () LPC-COMPUTER DE>]

Hi Folks,

as I'm new to the security scene I have to ask you a questions:

I've heard from a customer, that he believes, that all of his outgoing mail is read by someone using an email sniffer! 
My
question now is: has that to be server sided? I mean can anyone use this email sniffer or has he or she already hacked 
the
outgoing mail server?

How is this to be done?
What programms?
What procedure?
How would you do that?

Thanx in advance,

Jens Groh
Hostmaster / Security
LPC GmbH
Germany