[Exim] Checking for valid accounts in a mail from relays

I hope someone resolved this issues already and can give me some feedback on it.

I'm getting some my mail from a mail relay as forward *@mydomain.my to
myname@???. I.e. I have mapping - all mail for some
domain to one specific email address. Obviously with this scheme
envelope receiver's information is getting changed on the relay and I
cannot filter out invalid accounts any more with regular methods, but
I desperately want to do this because of huge amount of spam for
non-existing accounts I'm getting every day. Now it's handled by
mozilla client, but this is a simple case and I'd rather drop it right
after DATA segment without storing it in my imap box and need to apply
spam filtering after that.

In short - to pass there should be at least one legal destination in
DATA segment fields and I want to specify these legal destinations in
a file as list or pattern.

Please advise!

Thank you,
Andrey