I haven't had any problems with outbound deliverability. However, their inbound post-delivery spam controls are overzealous and categorize almost all mail from new correspondents as spam.

The only major problem with Zoho is that they host mail inside US jurisdiction so it's vulnerable to NSA abuses.

Mailbox.org (based in Germany) is a possible alternative for people who want/need to keep their data further away from American commercial espionage operations.