sophos licensing - one user or per "address"?

[Furnish, Trever G]

I'm curious - for those of you using sophos on a mailscanner system acting
only as a relay (not a "mailbox server"), did you license it based on the
number of recipients for whom you would deliver email or did you get a
single-user license (or something else I'm failing to imagine)?

Considering that we're talking about a mail relay, I personally think it's
silly to license a package based on the number of destination addresses
being protected by the product, given that the number includes not only your
internal users but also the people they send email to (assuming that you
filter outbound mail as well as inbound mail).

Sophos sales rep would obviously like me to license based on the number of
internal users I have on my destination system, which seems rather
ridiculous.

-t.