We have a dedicated abuse monitoring team who work on mechanisms to control misuse of our system thereby reducing the chances of our sending IP getting blacklisted. In addition, we recommend users to configure proper SPF & DKIM records for better delivery of their outgoing mails.

Could you please describe the specific issue that you faced along with the complete bounce message or screen shot, by writing an email to support@zohomail.com. This will help us to investigate further. Thanks. ^SP

Thanks for the heads up. I'll move to Google Apps on the first sign of trouble.