The tag is present in Lintian version 2.104.262. That is the most recent version we know about.
This package has a relationship with the mail-transport-agent or default-mta packages but does not specify the default-mta as an first option.
default-mta and mail-transport-agent should only ever be in a set of alternatives together, with default-mta listed in the primary position.
Please rearrange the dependencies such that default-mta is listed first.