E-mail communication to one certain e-mail address does not work

E-mail communication to one certain e-mail address does not work

If I write an e-mail to ***@web.de, the receiver does not receive my e-mails. Also not in his SPAM folder.

Same vice versa, I don't receive his e-mails in my zoho inbox (also not in my SPAM folder).

All other e-mails on both sides seem to work without problem. What is wrong there?

Thanks for your help!

Kim

P.S.: We can communicate without any problem if I go through my google account.