BT mail authentication problems.
I have been trying to get a friend's BT mail to send successfully. The BT helpline had got them to change passwords numerous times, but this seems to have just made for multiple password entries in the Keychain. After deleting all BT Keychain entries and re-installing BTinternet.com in the Mail app several times, it still refused to send.
The Apple System Preferences insists that the old BT/Yahoo mail, be installed, in spite of BT's saying it has been moved to their new system.
Then there is the problem of SSL, to enable or not. With this too, Apple seem to have diverged from the BT stated methodology. I suspect that the recent hacking of SSL may have something to do with this...
On noting that iCloud uses a Token system, I tried changing the authentication methods to TLS and other keys in the drop down list. The mail that had been stuck in the Outbox, suddenly went, so I am not touching it more, as playing with somebody else's computer is quite stressful at the best of times...
Here are some screen shots of the settings. Perhaps Apple will get round to telling BT how to set things up to their liking, and vice versa. I would dearly like to know too.
Incoming IMAP Server
Outgoing SMTP Server
Outgoing SMTP Server (Advanced)