The difference between email and fax is that, when sending a fax, your machine established a connection with the receiving fax machine by a "handshaking" procedure, under which, by exchanging messages, the faxed document would only be sent after the sender machine knew it was connected to the recipient machine. At the end of the transmission, a confirmation that included of the number of pages received and the telepone number of the recipient would be received. This was accepted in legal proceedings as evidence of delivery .
Conversely, email works on the "store and forward" principle, where a message is divided into data packets that have the address of the recipient. These packets are sent over the network from node to node. At each node a received packet is temporarily stored and then forwarded to the next node, and so on until it (hopefully) reaches the recipient, where all the packets are re-assembled to re-create the original message. Different packets of the same message might well travel via different routes if the network is busy, and sometimes messages get lost in transit and never get delivered. That is why email is not normally considered a secure delivery method, unless the recipient emails back confirmation of receipt.
At least, that was the situation when I retired a decade ago. Important documents, especially those associated with legal deadlines) were usually sent by email (for conveniently providing client with electronic copies of documents), fax (to positively establish receipt) and post (to insure against corruption in fax or email transmission).