Zoho mail server refusing connection (Socket error code 10061)

Hi,

A client tried to email and for some reason the zoho mail server appears to have refused the connection. I looked at the server response and it was "Socket error code 10061"

Is there some way to solve this problem? Is something misconfigured on the Zoho server? Or is there something I can do on my end?

Obviously this is a major problem if it recurs because it affects my business.

Thanks for any insight or help.