Hello everyone,
the following Error comes up at on clients Outlook:

that means in english
"The Message you are about to send is RTF-coded. This Coding is not supported.

Be aware that attached Schedules are not supported

A possible solution is to delete the attachements and send the mail again"

But The Messages aren't in the outgoing Folder. They are in an normal mailfolder for old messages.
there are no files attached and unfortunately, there are suddenly a huge number of Mails, so that deleting them will open the next mail in this folder with the same Error.

and not all Mail are suposed to be deleted.

There is no Problem in the webfrontend.
Does anyone have an Idea how to solve that Problem?