There are a number of Usenet groups devoted to electronic-mail technical issues:
the ELM mail system.
The Rand Message Handling system.
Multipurpose Internet Mail Extensions.
General discussions about computer mail.
The Mail User's Shell (MUSH).
the BSD sendmail agent.
the smail mail agent.
Mail in the uucp environment.
The following is a non-inclusive set of books that will help...
from O'Reilly and Associates is the definitive reference on sendmail-v8 and sendmail+IDA. It's a ``must have'' for anybody hoping to make sense out of sendmail without bleeding in the process.
from Osborne is a fine reference book that explains the various services available on Internet and is a great source for information on news, mail, and various other Internet resources.
from Olaf Kirch of the LDP is available on the net and is also published by (at least) O'Reilly and SSC. It makes a fine one-stop shopping guide to learn about everything you ever imagined you'd need to know about Unix networking.
Also worth mentioning is Chris Lewis' periodic posting on unix e-mail software, which is available on ftp://rtfm.mit.edu/pub/usenet/comp.mail.misc as the files named ``UNIX_Email_Software_Survey_*''. An HTMLized version is at http://www.faqs.org/faqs/mail/setup/unix/. At time of writing in 2005 this posting has not been seriously updated since 2000, however.
There is no longer anything special about configuring and running mail under Linux, relative to other Unixes. Accordingly, you almost certainly do not want to be posting generic mail-related questions to the comp.os.linux.* newsgroups.
Unless your posting is truly Linux-specific (ie, ``please tell me what routers are already compiled into the SLS1.03 version of smail3.1.28'') you should be asking your questions in one of the newsgroups or mailing lists referenced above.
Let me repeat that....
There is virtually no reason to post anything mail-related in the comp.os.linux hierarchy any more. There are existing newsgroups in the comp.mail.* hierarchy to handle all your questions.
If you post to comp.os.linux.* for non-Linux-specific questions, you are looking in the wrong place for help. The electronic mail experts hang out in the places indicated above and generally not in the Linux groups.
Posting to the Linux hierarchy for non-linux-specific questions wastes your time and everybody else's...and it frequently delays you from getting the answer to your question.