When I try to send emails from zoho mail to people with email addresses ending @outlook.com and @hotmail.com (and a few others), I get a 'delayed' automatic email and then a few hours later, an 'undelivered' automatic email.

This has started a few months ago and is becoming a significant problem.

There is no problem with the recipients email as they are getting messages from other people OK.

I could really do with help to fix this as it's making me think that Zoho is faulty and I should move elsewhere. This would be a terrible shame after so many years of faultless service.