If you are only sending to zoho, use their SMTP server. It won't allow you to relay (domains other than what they serve), but ANY SMTP server that hosts the domain you're sending to will allow you to use it to send to email addresses it hosts.
Go to a command prompt
Type "nslookup"
Type "set type=mx"
Type "zoho.com"
You'll get:
Non-authoritative answer:
zoho.com MX preference = 50, mail exchanger = mx3.zoho.com
zoho.com MX preference = 20, mail exchanger = smtpin2.zoho.com
zoho.com MX preference = 10, mail exchanger = smtpin.zoho.com
You can use any of those 3: mx3.zoho.com, smtpin2.zoho.com, smtpin.zoho.com