On a linux box what are all the possible services of sending a mail,i can think of the service like sendmail,Qmail..
And how to restrict all possible ways of sending out mails from a linux box (RHEL5)
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The list of potential ways to send mail out is too long to list here. The standard programs are sendmail, exim, postfix, and qmail. There are a number of SMTP relay programs which can be used to send mail. Many languages have library routines that can be used to send mail as well.
You can may it difficult by creating iptables rules to prevent access to the SMTP related ports; SMTP (25), SSMTP (465), and Submission (587). Blocking these ports will shutdown normal routes for email transmission.
Once you have blocked the direct access to mail servers there are a wide variety of other ways to send mail including WebMail, port forwarding, and remote sessions. These routes are not normally used by any of the programs or languages. However, if someone wants to subvert efforts to block email transmission they may use these techniques.
The only surefire way to prevent sending mail out from a linux box is to disconnect it from the network and keep it disconnected. This also applies to other Operating Systems.