Incorrect email address [email protected]

Whenever I publish a new post, I get email back over the following day alerting me of the failure to deliver messages to addresses with the format “twitter.<accountname>@example.com” or “facebook.<accountname>@example.com”. Please note that the domain of the address is literally “example.com”. Now, the subscribers concerned have NOT entered their email addresses with “example.com”, so there is clearly a default value that is being used, either by WordPress itself or by a plugin. But I am completely unable to find the source of the error and therefore cannot figure out how to correct it. I have scanned the code and have tried to find it in the database, to no avail.

Can anyone help?