Zoho mail sent from iPhone 5 sitting in outbox

Zoho mail sent from iPhone 5 sitting in outbox

Everytime I try to send an email from the Zoho account on my phone, it says "Unable to Send Email" and that it was rejected by server.

I am receiving emails fine though, on my phone. And I was able to send the email from the web client.

What can I do to fix this?