SMTP server would not relay a message

When setting up an IMAP/SMTP account, I get this message:

“The SMTP server would not relay a message. You may need to authenticate. (SMTP) View Log”

How do I get past this? I have no problem using this account with Evolution, Geary, Thunderbird, etc.

(By the way, this is with the latest version of Mailspring, 1.10.3)

Can it be that no one else has a problem with this? It’s a show-stopper for me.