Hello, I am trying to get a grasp of how t.37 Off Ramp faxing works. We have OnRamp working fine, where by a dial-peer is matched to an incoming fax number and passed to a mailbox. How does it work in the reverse?

If I put FAX=123456789@fax.mail.com in the to field of the mail client, how does Exchange know to relay this message to the router (3845)? Where does the address mail domain name come from?

Here is my config -

fax receive called-subscriber $d$
fax send transmitting-subscriber $s$
fax send left-header $s$
fax send center-header $t$
fax send right-header Page: $p$
fax send coverpage enable
fax send coverpage email-controllable
fax send coverpage comment NZ Cover Page
fax interface-type fax-mail
mta send server 10.100.0.160 port 25
mta send subject "Facsimile Transmission"
mta send origin-prefix Company Fax
mta send postmaster aklmainfax@domain.co.nz
mta send mail-from username CompanyNZFax
mta receive aliases [010.100.000.160]
mta receive aliases .domain.co.nz
mta receive aliases .domain.com
!
application
service fax_offramp flash::app_faxmail_offramp.2.0.1.1.tcl
!

dial-peer voice 8888 mmoip
service off_ramp out-bound
information-type fax
incoming called-number 77....... - Can you explain this please, is the router waiting for exchange to relay a number with 77 prefix then fax number?
image resolution standard
!
dial-peer voice 8889 pots
max-conn 10
destination-pattern 77.......
port 0/0/1:15
prefix 77

Thanks in advance

Brendan