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

Domain\Username
Enter your full email address, such as yourname@yourdomain.com

Password
Use the password that you use to access your email.

Server
Enter: zimbra.protectedservice.net

Port
Enter: 993

Security type
Select SSL/TLS or make sure the Use secure connection (SSL) checkbox is checked, then tap Next.

Outgoing SMTP Server Settings

SMTP Server

Enter: smtp.protectedservice.net

Security Type
Choose TLS

Port Number
Enter 587

Tap Next.

Tap Sign In.