It seems that many people have the same problem. Some ZOHO staff have reported that the problem has been identified and solved. But at least for me it is still continuing.
I am using ZOHO mail via a Thunderbird Mail Client on a MacBook.
The problems started about 2 months ago and have continued intermittently.
When sending emails the connection to the ZOHO server to relay the email is extremely slow, and often I get a timed-out error message.
As I said it has been going on for at least 2 months, and sometimes improves to hit me back with a vengeance.
Not being able to rely on the email service is extremely annoying.
Even more so because this has been happening for the past couple of months.
And on top of that ZOHO staff has been slow/silent when responding to issues raised.
And to top it all off, ZOHO staff has on various occasions reported that the problem was been identified and solved! Incorrect.
I'll give up and will take my mail service somewhere else.