I have had a paid Zoho mail account since April, which worked just fine.

But this morning I find I cannot send any mails, because I get this error message: 

I've tried reporting formally to Zoho Mail support and also calling their phone numbers, but I haven't received any response so far. 

Looking at the forums, I found some people reporting error messages like this when they are using Zoho as the front end to another mail supplier - for example this one: https://help.zoho.com/portal/en/community/topic/motivo-sun-security-validator-validatorexception-pkix-path-building-failed-sun-security-provider-certpath-suncertpathbuilderexception-unable-to-find-valid-certification-path-to-requested-target

That is my situation exactly.  I use a yahoo.com email address as my primary address, and Zoho was configured to send email from my Zoho account using that yahoo address.  I tried changing this setting to send via Zoho smtp, and problem disappeared. 

So I think this looks like it is something to do with Yahoo changing some security setting at their end.  But can anyone from Zoho confirm this, and if nothing else could we please have a rather more helpful error message than this one? 

Thanks