S0 n1c3, found out, that, if you have free GMX mail, you can use imap Protocol Server. for Mac:
- create new Mailbox, use a wrong password.
- After failing the test, select on the next screen the imap protocol and change the imap server to “imap.gmx.net” and user your correct password.
- voilà! now your email read status & deleted emails will be in sync if you set up the imap mailbox on every device.