The tag is present in Lintian version 2.104.318. That is the most recent version we know about.
This package has a relationship with the mail-transport-agent virtual package but does not specify the default-mta as an alternative.
default-mta and mail-transport-agent should only ever be in a set of alternatives together, with default-mta listed first.
Please add a "or" dependency on default-mta before mail-transport-agent.