Unable to send message;Reason:java.io.IOException: Host not reachable.

Unable to send message;Reason:java.io.IOException: Host not reachable.

Hi there, I'm getting the following error when sending an email:

Unable to send message;Reason:java.io.IOException: Host not reachable.

I'm sending an email from a user@zoho.com email address to myself at the same address. My POP accounts are working fine. I'm just getting this error on the Zoho email address.