Though parts of this post may be relevant, the original post was made for cPanel 10, so not all of the information you find here may be accurate. This post has been left here for reference purposes only.
This is the recommended mail configuration for optimal server performance, disk space conservation, and spam prevention.
The default address is disabled and set to :fail: for all shared hosting customers, so this has been done for you.
Resellers have the default address option enabled. We recommend you disable it from your feature manager so that your new accounts are setup to :fail" automatically, then they cannot be edited by your users.
The default setting for new accounts is:
:fail: no such address here.
This will bounce all improperly addressed emails back to the sender, instead of delivering it to the default inbox, which is usually never used. Disabling this feature in the feature manager assures that your end-users cannot change this setting. :fail: tells the server to bounce the email, "no such address here" is sent in the bounce so the sender knows that they have sent the email to an invalid address.
Spam Prevention via Spam Assassin:
We recommend you enable spam assassin, but keep the spam box, disabled. You should use the configuration wizard and set the threshold to 5 as a starting point, and have spam assassin rewrite your subject lines for mail tagged as spam. We recommend you you give it 1 or or 2 weeks where you can adjust the spam threshold higher or lower to "tune" the filter to your preferred level. This helps you avoid "false-positives," which means mail tagged as spam, but is not really spam. False positives can occur if your spam threshold is set too low. Once you have a comfortable spam threshold, you can now configure your mail server to delete these emails as they arrive, instead of delivering them to you.
1. To automatically filter and delete email tagged as spam, click on the mail filters option in cPanel, under the mail icon.
2. Click "add filter" then read the "hint" for Spam Assassin, then apply it. This will delete all mails flagged as spam.
You will see a considerable reduction in Spam to your inbox, and you will also see that you have eliminated a lot of wasted space taken up by junk mail.
Need more? Try this one on Mail Forwarding