I know there are other people using Zimbra.
Adding as "Exchange" into Aqua Mail won't work, Aqua uses EWS (Exchange Web Services) and not ActiveSync.
IMAP should work.
Network related error messages in Aqua Mail are "geniune", it reports what happens, doesn't make them up.
Please try using Manual button (after entering your email and password) and check server name, encryption type, and port # to make sure they're correct for your server.
Maybe the server has IMAP / SMTP disabled, too?