T.37 is an ITU standard for sending FAX accross IP networks in a store and forward mode.
FAX messages are sent as MIME encoded email messages using SMTP.
There are equivalent IETF RFCs.

Cisco whitepaper on T.37

Native Windows 2000/XP/2003 driver for T.37 client

See Also

• FAX
• FAX over IP
• T.38
• Steve Underwood Faxing over IP networks Article - excellent explanation of issues