In Confluence 2.5.2 and older it is possible to disable mail archiving by turning the shared mode on.

Since release 2.5.3, Confluence doesn't have support for "Shared Mode" any more and there is no alternative method for turning the mail archiving off.

In case that the mail archiving feature is not desirable, it is important to have an option to disable it globally. This is especially true for large deployments where site administrators delegate space admin privileges to large number of users who should but might not adhere to site rules.