The tag is present in Lintian version 2.109.6. That is the most recent version we know about.
This package has a relationship with the default-mta virtual package but does not specify the mail-transport-agent 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 mail-transport-agent after default-mta.