Undeliverable:**Message you sent blocked by our bulk email filter**

Hello everybody,

Some users in my company are getting this message regulary
{ Undeliverable:**Message you sent blocked by our bulk email filter** ) even if they are not sending any e-mail.
I tried to add the domain to the blacklist. But they are still getting thoses messages.

It looks like someone is bombarding my domain of this type of spams.

can anyone helps me to resolve this problem?