Sending with own email now failing

Everything has been working fine, but since the last time I sent an email I am getting the error message show below. I can send and receive email from the server that is using MailPoet, from the command line with openssl, but the credentials do not work in the plugin. This used to work fine.

The message is: The email could not be sent: Failed to authenticate on SMTP server with username “–redacted–” using 2 possible authenticators. Authenticator LOGIN returned Expected response code 235 but got code “535”, with message “535 Incorrect authentication data

Any ideas?