Because the MMP serves as a proxy for the Message Store, it needs to protect access to end user data and guard against unauthorized access. User IDs and passwords provide basic authentication capabilities. In addition, you can use client access filters to limit user login to specific domains or IP address ranges. SMTP Authentication, or SMTP AUTH (RFC 2554) is the preferred method of providing SMTP relay server security. SMTP AUTH allows only authenticated users to send mail through the MTA. For more information, see Enabling Authenticated SMTP.
Locate the MMP on a different machine (or under a different userID) in front of your POP or IMAP services. You can have front-end machines with just MMP and MTAs, and then have a physically secure network between those front-end machines, the mail stores, and the LDAP servers.
Special security considerations need to be given to Messenger Express access to the Message Store when your users are logging in from the Internet. In general, you want to make sure that the stores are separated from the outside world by a firewall. Like the MMP, the Webmail Server supports both unencrypted and encrypted (SSL) communication with mail clients.
Regular monitoring of log files can protect against unauthorized access.