On observing we identified an abusive usage pattern using our Zoho Mail service, which resulted in sudden surge in our outgoing email processing servers and our sending IP address getting throttled by few email service providers. 

Our technical team is working with high priority in clearing backlogs and taking action to ensure no such delays in future. 

We will keep you updated as we progress. 

Apologies for the inconvenience caused.  

Regards,

Zoho Mail Team.