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)