Old emails become "unread" again

Hello,

On my Samsung Note 3, stock Android 5.1, I use the standard Email app for managing my 5 e-mail accounts.
4 of them work perfectly, but 1 account does a strange thing.

Situation: all e-mails are downloaded and have been read. A new e-mail (on that 1 strange account ) comes in, I read it, I delete it, and by the next synchronization, the OLDEST email ( from that 1 account ) which was already previously downloaded and read, comes in as a new email and is marked unread again. If I delete 2 new mails, 2 old mails reemerge. Etc. etc.
Sometimes, for no apparent reason, lots and lots ( sometimes more then 100 ) of old emails reemerge as new mail and are marked unread again.
Every time this happens, I have to scroll all the way down to mark them read again.
All accounts are setup the same, but only this one ( the standard mail account ) shows this behavior.

Anyone have an idea what this is and/or know what to do about it?

Thanks in advance!
Grtz Tim

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Old emails become "unread" again