Bug#495844: package deviates from standard mail-transport-agent dependency.

[Andreas Henriksson]

I see your problem, but on the other hand I don't think the current dep
expresses it correctly. For anyone who already has an mta installed (which
is a pretty high risk) the postfix | m-t-a will not do anything like
you wish for.

I don't know how the package management tools handle this, but I would look
into how "Dep: exim4 | m-t-a, Recommends: postfix" would be resolved.
That would in my mind more clearly express what you are after and since
the package management tools nowadays installs recommends by default chances
are the user will actually end up with a configuration you guys support
even if they have a previous mta installed.

--
Regards,
Andreas Henriksson