MTA's are responsible for the delivery of Internet mail using the SMTP protocol. Sendmail is a very complex program and has been susceptible to numerous vulnerabilities. The best probable way to secure Sendmail is to stay current in your version. Several alternative MTA's exist. QMail is generally considered more secure and has good support for Virtual domains.
POP is a rather simple protocol that retrieves messages from an user's inbox.
The user's mail is stored locally with their mail client. One way to improve security on a system is to deny shell login abilities to POP account users. If their password is stolen, the cracker might be able to compromise the user's email, but not the system. IMAP is a protocol which allows user's to manage a virtual mail store on a remote system. User's can have their Inbox and mail folders all stored on a remote mail server.