This has happened also in my gmx account and I'm of the opinion that their server puts it there. Gmx and mail.com seem to be both parts of the same umbrella organisation.

Sheesh. Most of my IMAP based accounts have folders i never use and don't know the purpose of. IMIP? What's that? Why get heated up over another one?

I'm sad that the providers have no clue about this.

One poster had a theory that Gmail have bought gmx/mail and the folder is some form of preparation for integration with gmail: re gmail. I haven't been able to find any corroboration of such a merger, takeover or buyout.

Thanks for your thoughts. I still believe this folder was created by Mail.com but they say no. If it was created in one of my device's it would also show in Mail.com. And I would be able to delete it. I asked about GMX and they said it was their company. To be in two different devices will two different operating systems and two different mail programs there is only one common thread, that is Mail.com.

The first thing here is to acknowledge that it is clearly a server folder. It is unfortunate the mail.com apparently do not have staff competent enough in mail to establish that fact.

Hidden folders are as, Zeno says, a fact of life in the IMAP world. right click your account, select subscribe and unsubscribe from anything that looks useless. I had to do it with a yahoo folder that had Yahoo chat history in it. (that is not mail)

I do not see the folder in GMX, But RegMail would be an abbreviation of registration Mail. I wonder if it appears after you request a password reset, or is a folder set aside for incoming registration emails. from other sites. Just speculating. The only ones that really know are Mail.com.

Matt, Thanks for the idea. I changed my password in Mail.com but the rogue folder still lives. I guess I will just have to endure.

Just to comment that this isn't specifically a Thunderbird issue: there's a posting here where someone has exactly the same problem with Apple Mail -- again, with a GMX account http://superuser.com/questions/1043556/odd-new-regmail-mailbox-in-apple-mail-9-2

I am starting to see that Thunderbird is not the problem. Mail.com insisted I must get help from Thunderbird because it was not a Mail.com problem. I feel they are not on track with who really is the problem. There are too many indicators which point to a Mail.com/GMX issue and none which point at Thunderbird. Thanks everyone.

Just unsubscribe from it. Yesterday I did forget to mention that your may need to change the setting in your account setting under advanced in Server settings to only show subscribed folders. Tidies up those rogue folders. They obviously hide it, so you can to.

Thanks