You can also use access control mappings to prevent people from relaying SMTP mail through your Messaging Server system. For example, someone might try to use your mail system to relay junk mail to thousands of mailboxes on your system or on other systems.
By default, Messaging Server prevents all SMTP relaying activity, including relaying by local POP and IMAP mail clients. If clients do not authenticate by using SMTP AUTH, as described in Enabling Authenticated SMTP, and attempt to submit messages to external addresses via Messaging Server’s SMTP server, their submission attempts are rejected. Thus, you will likely want to modify your configuration so that it recognizes your own internal systems and subnets from which relaying should always be accepted.