In Outlook for iOS, go to Settings > Add Account > Add Email Account.
Enter your email address. Tap Add Account.
If you see the Select your email provider screen, choose Exchange
Note: If Exchange does not appear as an option, you cannot add your zimbra mailbox to the installed version of Outlook. Check for an update to the Outlook application or use the iOS built-in mail application.
Enter your password and toggle on Use Advanced Settings.
Enter the server settings as shown below.
Incoming Server Settings
Enter your full email address, such as firstname.lastname@example.org
Use the password that you use to access your email.
Select SSL/TLS or make sure the Use secure connection (SSL) checkbox is checked, then tap Next.
Outgoing SMTP Server Settings
Tap Sign In.