Penetration Testing: Identify the make and model of a Mail Server

["Doug Fox" <dfox168 () hotmail com>]

One can use NetCraft (www.netcraft.com) to identify a web server if it is 
Appache, IIS, etc.
How can one identify a mail server behind a firewall, be it Exchange, 
GroupWise, or Lotus Notes?
nmap or nessus helps identify if a mail server is available through tcp port 
25.
Any info is much appreciated!

Regards,

DF

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