Recognition of T.37 sender email address

When using the T.37 option Set User From Envelope Sender if the user is not getting detected correctly you'll want to check the headers of the email being sent.  

The Faxfinders use the Return-Path field and not the From field for matching up the user.  Typically these addresses will be matching, but not always.  

If the detected email address is unknown check with the sending email client/mail host to make sure that the Return-Path is set to match the defined users in the Faxfinder.  Also check that the case sensitivity matches.