I have followed these instructions,
Running Kerberos with Zimbra Collaboration Suite - Zimbra :: Wiki with the exception that my kerberos server is external, not local. I can login to the web interface with my kerberos password and get authenticated fine, but when I try to use either Mac's Mail or Thunderbird and GSSAPI/Kerberos5 authentication they are returning that "The mail server does not support GSSAPI authentication"