Zoho Calendar soft bounce on @hotmail.com and @yahoo.com email addresses

Zoho Calendar soft bounce on @hotmail.com and @yahoo.com email addresses

Hello, our Zoho calendar recently does not send the calendar invites to emails with hotmail and yahoo domains and comes back with a "soft bounce". other domains like Gmail works fine. Also sending "email" to the same emails to the above domains work well (only calendar invites has a problem. 
We've been experiencing this issue only in the past couple of week. It was fine before that. 
Any ideas?
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