The   Barracuda Network, which relies on customer reports and complaints from all their equipment worldwide has marked domain zcsend.net [Zoho Email Sending Domain] and is flagged as a known spammer. Well, not them exactly, but their network of worldwide clients who used their service to send email in such volume that the complaints led Barracuda to add it to their global "block" list.

By contrast, Barracuda trusts emails sent by Constant Contact, Vertical Response or MailChimp enough that they're automatically "whitelisted" worldwide. The difference is in what the two vendors accept as input. CC auto-checks and gives you an estimated spam score before it allows you to send anything. I don't know much about Zoho but I suspect they don't do this, instead allowing the content to be entirely in the hands of their clients.