The tag is present in Lintian version 2.109.24. That is the most recent version we know about.
The contact's mail address either loops back to itself or is known to bounce.
Loops happen because an address is
firstname.lastname@example.org. Bounces happen when the
receipient, typically a mailing list, is known to bounce mails.
The mail address must accept messages from role accounts used to send automated mails regarding the package, including those from the bug tracking system.
Please refer to Debian Policy Manual section 3.3 for details.