so what are some EMail services?

Email is sent thru a network of email servers. this is knows as Mail Transfer Agents;

there are several MTA programs available for linux, but the most comonly used program is called SENDMAIL. yet, another popular program is called POSTFIX

postfix is designed to be compatiable with sendmail and the user interface is kinda the same. the internal workings of POSTFIX are somewhat different from SENDMAIL because POSTFIX was designed to be faster, more secured with more easy administration.

do you know of any other MTAs?