I bet they spoofed the mail header. This is EXTREMELY common for Spammers. Its happed to all of us. If you havent seen emails “from” YOUR Domain yet, you will.
If your are talking the signature, well spammers arent high on ethics. They could have just as easily signed it:
DictatorGeorgeWBush.com
The emails were sent by a spammer promoting this service:
xxxhttp://www.1on40.com/99689666
The trailing number is either:
a: A tracking link used by 1on40 to see which campaigns/headlines/text is drawing click throughs
b: An affiliate tracking link. A spammer is promoting 1on40 and they arent aware or dont care that this affiliate is doing this.