There's 3 ways to do IMAP:
  • 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
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.