Avoid Sendmail. Use
Postfix. I fully agree with the quote from their website: Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Thus, the outside has a sendmail-ish flavor, but the inside is completely different.
Of course that takes care of getting the mail to the box, then you need something to be there to answer the POP/IMAP requests. I personally use
Courier for my IMAP, and whatever SuSE uses by default for POP. (I keep them seperate because I have Courier set to use Maildir format mail, much faster for IMAP, whereas I use the old mbox format for POP. It also allows people to use POP mail, then turn around and use IMAP as an archive without the two clashing).
Definitly look into
Webmin to manage your box. It has saved me so much time, and presents a unified configuration interface to all the different Linux distributions.
And to answer what Linux distro to run, go with
SuSE Pro if you don't want to download much, and prefer to install off the CD. On the other end of that, go with
Debian if you want one of the easiest autodownload and install systems.