is there real things I can do?
Assign the customer a /32 from one of your address
blocks so that they can use it on their mail server.
This means the mail server will not be multihomed
but at least it will work. Maybe they can also get
another /32 from their other upstream provider.
In fact, you could also offer to host their mail
server in your network which would accomplish
the same thing.
--Michael Dillon