Federal Law has specific limits on sending junk faxes.   47 U.S.C. § 227,

The sending of unsolicited faxes violates Federal Law and subjects the sender to statutory damages of $500, which can be tripled if the court finds the violations to be intentional.

Identification Required on Fax Messages

Federal Law required specific identification to be included on faxes

• Date and Time of transmission
• Sender's identity
• Telephone number of the fax sender and fax number of the sender