The T.37 fax standard, set jointly by the International Telecommunications Union (ITU) and the Internet Engineering Task Force (IETF), is a sister standard to T.38 that contains recommendations for sending faxes over IP in a store-and-forward (or email) format. The T.37 fax standard was set before T.38 and is a simpler standard because it doesn’t involve spoofing. After all, since faxes are sent in a store-and-forward format, there are no timing issues and the standard doesn’t have to deal with questions like delayed packets and jitter. Faxes are transmitted as MIME-encoded (Multipurpose Internet Mail Extensions) email messages using the SMTP (Simple Mail Transfer Protocol). Here is a great tech note on implementing T.37: http://resources.otgt.com/acton/attachment/7088/f-0019/0/-/-/-/-/file.pdf
The one big drawback with the T.37 fax standard is the fact that confirmations of successful transmissions or error reports are delayed, just as email transmissions may be delayed. When emails are sent, the sender doesn’t know if the email will arrive at its destination in a few seconds, a few minutes, or several hours later. So, when a T.37 fax is sent, the same potential for delays ensues. The T.37 fax may be delayed and/or the confirmation may be delayed. Since many T.37 fax users wait for the confirmations to come to complete the fax, this method has not been as widely used as T.38.
Emails are handled with Tag Image File Format (TIFF) attachments only. The use of distribution lists means a single fax may be sent to multiple email addresses or fax machines. The T.37 fax is generally used with gateways so a T.37 fax may be used to send to any fax device and any fax device may reach a T.37-enabled fax device.
Initially, the T.37 fax standard was only used in Internet fax machines, but more recently has been offered as an option on fax servers like those from Sagemcom, Cisco or Softlinx.
In simple-mode, a T.37 fax utilizes the TIFF S-profile and limits fax transmission to widely used fax machine formats (e.g., standard-size pages and standard or fine resolution). Also, in simple mode, no confirmation of delivery is provided. In full mode, the T.37 fax standard includes extensions which ensure call completion through handshaking between sending and receiving devices. Delivery confirmation is also provided in full mode (but are still subject to delays). Extensions to the standard also support color, black-and-white and grayscale faxes.