When modifying MX Records to point to mx.zoho.com, my existing email addresses don't work - the emails bounce. I'm very concerned about this down time. I've waited more than 4 hours (the TTL = 14400 or 4 hours) but no change in DNS entries. I've read online some folks were out of email for an entire week. I can't afford that. If not working, I'll need to revert.

My question is -- how can I ensure this will work? Do CNAMEs have to change too? Or just MX? I've been using the MX toolbox to check and no updates still.

http://mxtoolbox.com/SuperTool.aspx