Problems with sending to certain servers

Hello. In the last few weeks, when sending emails, I have increasingly received the automatic response “Mail Delivery Status Notification (Delay)” with the notes “The following addresses had fatal errors” and “Message will be retried for 4 more day(s)”. Only to receive the message “Undelivered Mail Returned to Sender” after a few days. With the error messages “A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.” and “ERROR CODE :450 - 4.7.25 Client host rejected: cannot find your hostname.”

Among other things, I have problems with emails to the TLD "arcor.de" and "colter-gmbh.de". Receiving emails with these addresses is not a problem. It's all about sending. (And only from my Zoho address. It works perfectly with other email addresses (e.g. GMail).)

What's going on here? Why is that? How can this be fixed?

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