smtp errors with the settings that have been working

my smtp server is not allowing mail to send. here is the message i get when attempting to send new messages:

Sending failed:
Your SMTP server does not support The server responded: “Incorrect authentication data”. Choose a different authentication method. The server responded: “Incorrect authentication data”
The message will stay in the ‘outbox’ folder until you either fix the problem (e.g. a broken address) or remove the message from the ‘outbox’ folder.
The following transport protocol was used:
Do you want me to continue sending the remaining messages?

Authentication is set up as Plain.

anybody understand what is going on? what do information do you need, and how can i remedy this situation?