If have run sendmail for sometime with no problems. What you want to do is make sure you have the most current version. And, after it is set up, runn something like nessus against it, and it will tell you the holes and how to plug them. most of the problems with sendmail come from misconfigurations or loose restrictions which allow people to relay mail through your host, log onto the server from a telnet session, and generally gain some basic information about users or the sendmail version number. Always check the security bulletins on for the latest root-level compromises and other security hole (as you should for most services running on your servers)and you should be ok.
If you tighten things up, you have little problems. To support remote users you will most likely have to loosen your relaying rules. Qpopper, or any pop3 or imap package, should always require a password to be exchanged in order to retrieve mail any at all.
That being said, sendmail.cf is a bitch to edit. If you go with sendmail, I suggest using something like Webmin for daily administration tasks.Or use something like Qmail or postfix. i've heard a lot of great things about both those smtp servers.
If you are using sendmail or postfix, you can always use a web-based mail interface like neomail, or hack one together from perl or php.
Hope this helps some.