I'm not sure what you've read, but it sounds wrong to me.
MTA's (mail transport agents) simply deliver mail to a mailbox (say sendmail, postfix, qmail, etc). You then use a server to allow MUA's (the acronym escapes me for a moment, but think outlook, evolution, etc) to read the mail.
If you use a pop server, then generally the mail is downloaded to the MUA, whereas imap generally leaves it on the server. I use dovecot, but courier-imap is another.
Squirrelmail interfaces with the server to read mail from the web.
Using an MTA with an imap server on your samba machine sounds like it will do what you want.
Personally, I run postfix, dovecot and squirrelmail, and use outlook from windows machines to access mail. All mail stays on the server.
Have a look at
www.postfix.org I've also seen plenty of posts refer to
www.qmailrocks.or (at least I think that's it).
You can also run spam and virus filters in conjunction with the MTA to trap any nasties.
Rgds
Bill
Edit - note that if you want to pick up mail from, say, and ISP pop account, look at fetchmail, which then delivers to the MTA