Error sending messages: PHPMailer debug message: 1 -

When I send messages out previously I got a status message in watchdog saying that an error occurred or that the message had been sent to a user. Recently, I onlyget the message: PHPMailer debug message: 1 -

When I try to send the message via the Devel Debug option, the message is registered as OK.

I am reluctant to deinstall the modules messaging/notifications as I lose all user settings for these modules. Is there another way to resolve this?