It works by sending a single command: ID ("GUID" "1"), and then the
rest of the IMAP commands are the same as for any other server. The
Zimbra Desktop client also supports some fancy authentication scheme,
but this is unnecessary as you can just use encrypted password login
over SSL, and that should be about as secure.
I really do want to write an extension for this. I don't think it is
possible to send random IMAP commands from an extension, so it would
probably involve writing a proxy. In turn to support SSL would require
openssl on the client computer. This is generally not a problem on
linux, but it would be an extra download on windows.
On the other hand since this is only a few lines of code, I will see
if it can be included into the regular Thunderbird source, but it is