PowerMTA is an enterprise-grade email message transfer agent (MTA) software for sending high-volume, business-critical emails. PowerMTA can be run on your own server or in any public cloud such as Amazon Web Services or Microsoft Azure.
The Zimbra server is a dedicated server that manages all of the mailbox contents, including messages, contacts, calendar, and attachments. Messages are received from the Zimbra MTA server and then passed through any filters that have been created. Messages are then indexed and deposited into the correct mailbox.