[czaveri]

MTA, POP, IMAP and Zimbra are different things.

MTA might require you to use TLS and yet you would have configured Zimbra UI not to use it.

In the Admin Console of the Zimbra Instance, check if the MTA is configured to use TLS or not.

Here's where you should check:

1. Admin Console -> Global Settings -> MTA -> TLS authentication only

2. Admin Console -> Servers -> host.yourdomain.com -> MTA -> TLS authentication only

You should check the above if you are facing problems while "Sending" email. However, if you are facing problems while receiving emails, You should check the POP and/or IMAP tabs for Global and Server Settings.

For example, if you have TLS enabled for MTA and you face problems while sending email, you need to check that you have enabled TLS/SSL in your mail client. In your case, Outlook.