Folder internal data message
I've received a message: "DON'T DELETE THIS MESSAGE -- FOLDER INTERNAL DATA"
"This text is part of the internal format of your mail folder, and is not
a real message. It is created automatically by the mail system software.
If deleted, important folder data will be lost, and it will be re-created
with the data reset to initial values."
There are different reasons why this email may appear in your Inbox.
If you do receive this message, it is safe to delete and doing so will not affect the working of your email client.