Please check to make sure that the account's outgoing server has authentication (login) configured:
( unless this account uses OAUTH, such as Gmail / Hotmail / Yahoo in which case there won't be a password input field on any of the account setup screens )
For this long press on the account (on the app's main screen) -> account setup -> Manual
Overall, this kind of message can only come from the account's outgoing mail server, and if it's intermittent, then the settings are most likely correct, and it's most likely a mail server issue (bug / misconfig) and you should take this up with your mail service's support.
