undeliverable mail

[Beastie MRA]

Dear All.

For past few days, my MX receive thousand of undeliverable message
destinated for my non existent user at my domain.
This message source come from valid and well configured (almost) smtp
server on internet.
I'ts waste my internet b/w, cause my MX will reject with non existent
user message.
I'll try spamd on my firewall and greylist on my MX (postfix), but still
no effective, and i cannot block undeliverable
message as RFC rules

Is there any way i can fix this ?
Please help

regards
Reza