Identifying which MTA is running

I dont have knowledge on linux and how can i know which mail server is running on my linux box? Postfix/Exim? Btw Didn't found postfix folder in etc/, where can i find master.cf? .Any idea for modify cPPOP?

For your second question, use locate to find master.cf

updatedb
locate master.cf

Easiest way might be to get the PID of what's listening on port 25:

 # sudo lsof -i :25   
 COMMAND  PID USER   FD   TYPE DEVICE SIZE NODE NAME
 master  5664 root   12u  IPv4  13732       TCP *:smtp (LISTEN)

Then find out what that process is:

 # ps p 5664
 PID TTY      STAT   TIME COMMAND
 5664 ?        Ss     0:12 /usr/lib/postfix/master

And I'm running postfix. For sendmail, it looks like this:

 # lsof -i :25
 COMMAND   PID USER   FD   TYPE DEVICE SIZE NODE NAME
 sendmail 3445 root    4u  IPv4  12922       TCP localhost.localdomain:smtp (LISTEN)

Edit

Postfix config is under /etc/postfix , "sudo" might be necessary

most mail servers will identify themselves

justin@bert ~ % telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 xxxx ESMTP Exim 4.69 Sat, 09 Jan 2010 15:08:05 -0500