There's 3 ways to do IMAP:
Your zimbra install was providing the third of these, while your Mac Mail was looking for the second. You can either enable cleartext logins, configure Mac Mail to issue a STARTTLS, or turn on IMAPs and configure Mac Mail to go to port 993.
- IMAPs (port 993), where the whole connection is encrypted
- IMAP (port 143) with cleartext LOGIN
- IMAP (port 143) with STARTTLS before LOGIN to introduce an encryption layer