It seems that some ISP's, such as AOL and possibly AT&T and others, have run into a situation that - when someone tries to send an email, it's gets bounced back as a spam attempt by a company called SORBS in Australia, a spam blocking company.
Rather than try and explain what's going on, here's a link to a story about it on The Register.
http://www.theregister.co.uk/2010/10/07/sorbs_cockup/
Unfortunately, the IP address we use on our server apparently seems to be involved with this mess. So if anyone has tried to send an email and it gets bounced back with an error something like this:
----- The following addresses had permanent fatal errors -----
(reason: 550-Email blocked by SORBS - to unblock see)
Now you know why.
When will this get straightened out is anybody's guess.