In the past I had this happen to me when my company moved to a new location with a new ISP. The block of IP's that they assigned me were once used by spammers to send out a bunch of garbage emails. In return, these IP's became blacklisted by major service providers (AT&T included).
I don't believe your computer has been hacked, but your ISP assigned you an IP address that was once allocated to someone who used to perform actions that caused them to be blacklisted. The best thing you can do in this case is either request a new IP from your ISP, or send emails to AT&T and other email providers to unlist your IP from their blacklist.
Requesting a new IP from your ISP can be done in a few ways, depending on how your ISP works. The following are a few common methods to obtaining a new IP.
- Call them and request a new IP.
- Unplug your modem for a few minutes and plug it back in.
- Clone your MAC address on your router/PC.
Hopefully this clears up your question!