Bounce Error

Hello,
our client is getting some error e-mails regarding bounce processing. Is it coming from your plugin?

The message looks like this:
Invalid Login Details for Bounce Processing

Automatic bounce processing was trying to log in to the email account (username ‘xxx’ at the email server
‘xxx’) but was unable to.

It received the following error message: ‘Retrying CRAM-MD5 authentication after [AUTH] Authentication failed.’.

Thank you for your help.
Toni