How to check my mail system?

I don't know If I am using one of those, Sendmail, Exim, Qmail or some other system.

How do I check my mail system??

From serverfault $pageUsers[$entry.posts[0].author].name

• lsof -n | grep TCP | grep -i smtp
  

  That should let you know what's got the port open.

  Matt Simmons : whoops! Sorry! In that case, definitely use sysadmin1138's suggestion
  From Matt Simmons

• One good test is to telnet to port 25 of your mail-host and see what it tells you it is.

  C:\> telnet mailhost.mycompany.com 25
  Connecting To mailhost.mycompany.com
  220 Mailhost.Mycompany.com Microsoft ESMTP MAIL Service ready at Thu, 18 Jun 2009 21:15:06 -0700
  QUIT
  221 2.0.0 Service closing transmission channel
  

  That would tell you your mailer is Exchange of some kind.

  C:\> telnet mailhost.mycompany.com 25
  Connecting To mailhost.mycompany.com
  220 mailhost.mycompany.com  ESMTP Sendmail 8.11.7p3+Sun/8.11.7; Thu, 18 Jun 2009 21:17:26 -0700 (PDT)
  QUIT
  221 2.0.0 mailhost.mycompany.com 25 closing connection

  That would tell you it's probably a sendmail of some kind. Just google the result string you get, it should be clear what it is.

  Edit: If you're running it from either WinXP or the Windows 2003 server itself, telnet is found in C:\Windows\System32\telnet.exe. You can directly invoke it:

  C:\> c:\windows\system32\telnet mailhost.mycompany.com 25

  Graeme Donaldson : @mcfadder_09: I'm assuming you're attempting this from Vista/2008/7. You need to install the Telnet client as it's not installed by default any more. Install it via Control Panel / Programs and Features / Turn Windows features on or off.
  From sysadmin1138
• • Do you have physical access to this server?
  • Can you login?
  • Perhaps check the Start menu. The name of the mail server should be in there somewhere.
  • Open the Services MMC window. You'll see it in there somewhere.

  Enable telnet on the Windows machine you're on.

  From pcampbell

• What are you using as an email client?

  I'd start by looking at message headers. Open an email message you received from someone outside your company and look at Received: lines. Send an email out to Gmail (for example) and look at it (with "Show Original" if you do use Gmail). If your server is Exchange, there should be a line like: X-MimeOLE: Produced By Microsoft Exchange V6.5

  This isn't a sure thing, but there's often an indication of what mail server and MTA are involved.

  From Ward