[keithnz]

not sure I'm understanding? But fax sends data via a protocol, it knows it has sent by virtue of the protocol completing. The fax can keep trying sending and it will keep track of those faxs that have gone through vs those that haven't. Email doesn't have delivery receipts....it's either done by bolting it on in clients or various hacks used by spamme....errr..email marketing systems.

[ses1984]

Just because you get a receipt from the protocol doesn't mean it made it from the fax machine to the intended recipient. Folks who send faxes still regularly follow up with calls and emails, "just sent the fax did you get it?" and the answer is often "no, what fax?"