Macmail

Have just spent 2 hours with our mail and internet provider, Telstra bigpond who have tried numerous things and believe it is a compatibility issue with Apple now. They have setup a IMAP and POP account and both won't allow emails to be sent.

"This message could not be sent because your account does not have a preferred outgoing mail server"

It is running on MACOS sierra 10.12.6

We have deleted and added this 4-5 times now.

emails being received but none able to be sent.

Does anyone have any ideas?

Many thanks