[thelist] Spam from my domain

[April]

Hi All,

I'm trying to contact my hosting company about this, but they're on a 
skeleton crew for holidays... so who knows when I'll get a reply.

A few different email addresses on my domain are being used as the From: 
in a bunch of spam, starting maybe 20 minutes ago. They're not real 
users.. I had a catch-all address on, and started getting the hundreds 
of bounce messages in my inbox, which is how I knew.

I know that the From: doesn't necessarily mean it came from there... but 
how can I definitely verify that my website isn't being used to send 
them somehow?