Mail App receiving but not sending

I logged into my personal email on my MacBook and iPhone and I can receive emails but I can't send them. I believe it is to do with my authentication.

I get the error message "Local senders are prohibited to send to local recipients without authentication". I checked Apple's mail settings look up and it says I should have my outgoing server as:

Server port: 587

SSL/TLS: Yes

Username: (my email address)

Authentication: Password

Server Hostname: smtp.sasktel.net

As soon as I set the authentication to password, I get the error "unable to verify account name or password". I have changed my password on my account and reentered. I have deleted the account from mail and reentered and nothing. I know my username and password are correct but it won't let me put the authentication to password. The only thing that works is no authentication and then I can't send anything.

What do I do?