Odd new "RegMail" mailbox in Apple Mail (9.2)

A few days ago I had a new mailbox in Apple Mail, which I did not create but which is listed under my on-line account with GMX.

The mailbox is called "RegMail", it isn't on GMX and it doesn't delete. If I delete the mailbox from ~/Library.Mail/. it comes back. Nothing shows up under a virus scan and it contains no emails. Rebuild and sync do not get rid of it.

Anyone else seen this? How to get rid?

D

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