Mail not sending to some recipients

Mail not sending to some recipients

In addition to previously mentioned problems, Zoho mail is now not attempting to send messages to one of my recipients. (A chiark.greenend.org.uk address. I'm now using Thunderbird configured with Zoho, in order to avoid the RO header problem mentioned before.)

This is really not good!

Please could you reply swiftly to this and my previous problem report? If problems like this continue I'm going to have to leave your service, which would be miserable. But I need to know that when I send messages, they're going to get through.

Thank you!