Hi Deniro,
First of all Merry Christmas to you and all in your circle of family and friends.
Let me add one thought to your situation.....it appears to me to be a legitimate notification on the face of it, and therefore merits at least a modest attempt to reconcile.
I'm going to make a few baseline assumptions on your behalf....
Your Macintosh computer(s) are well maintained and up to date with security updates.
Your use of your Macintosh computers is focused on the brighter side of the internet rather than the darker side where illegitimate activities run wild.
You are bringing this issue to us for additional help and guidance because you are truly concerned that your personal computing suite might possibly actually be producing the underlying spam called out in the ATT notification (which might also be some sort of illegitimate correspondence in its' own right).
So with all the above in mind, I personally would disregard the notification and get on with your normal holiday routine.
The one outside factor that might actually produce the spams supposedly originating from your assigned IP address might be some other "smart" device installed on your home network.....such as a NAS or even another networking device such as a WiFi extender or second access point....these additional network components are normally controlled by an embedded processor running some variant of Linux and potentially they can be and might have been hacked in the past.
Temporarily removing them from your network to reset them to factory defaults, installing the latest firmware updates and then reconfiguring them to your network should remove any further potential for them being responsible.
