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

Hello all, I am getting a lot of users that are getting this message every morning ( Undeliverable:**Message you sent blocked by our bulk email filter** ) but they are not sending any e-mail this is happening at night while no one is here do you think this is some type of virus or spyware?? Re: ..an advertising method worth thinking about is in all of our subject lines any suggetions or advise?

Thanks