E-mail Virus?

I've been averaging about 1000 pieces of spam mail a day as of the last 2 weeks. At least half of those come with a subject line of:

Mailer-Daemon
Undeliverable Message
Delivery Status Notification (failure)
Barracuda Spam Firewall

I don't use a spam filter for fear I would not get important e-mail. And I do use the received receipt function in my outlook 2002 to verify that clients recieve my e-mails. However, the addresses in the above subject lines are not ones that I have ever heard of or have in my address book for that matter. So I don't know what is "undeliverable".

Do you know of any viruses out there that would cause this?
Is this a Spam issue? If so, what filter do you recommend? At this rate, I'm missing the important messages because my box is filling up with all the above junk anyway.

I've run norton anti-virus - nothing
I've run adaware - nothing
I've run Mcafee stinger - nothing
I've run panda - nothing
For some reason, trendmicro's free program keeps freezing up on me midscan.

Helllp!

Dawn