I recently made the huge jump from Snow Leopard (10.6.8) to High Sierra (10.13.6) only to find that Apple mail would not take my account online. I fiddled for days until I was successful. However, that was only half the battle as Mail insisted that it couldn't connect to the outgoing server. I finally stumbled upon the solution in server settings by checking the box allowing Mail to automatically manage the connection as well as use TLS/SSL. Mail assigned 465 as the port and I am now able to send messages. Incidentally, authentication is none.
