I hope I'm asking a stupid question.

Is the IMAP proxyauth feature enabled by default? I'm trying to migrate from Sun to Zimbra using '--authuser2 admin', but I get this: " BAD parse error: command not implemented". When I use imapsync I get the same result.

Here's output of an interactive test using telnet:
* Sun Messaging (works)
Code:
 telnet imap 143
 Trying x.x.x.x...
 Connected to imap.
 Escape character is '^]'.
 * OK [CAPABILITY IMAP4 IMAP4rev1 ACL QUOTA LITERAL+ NAMESPACE UIDPLUS CHILDREN LANGUAGE STARTTLS XSENDER X-NETSCAPE XSERVERINFO AUTH=PLAIN STARTTLS] Messaging Multiplexor (Sun Java(tm) System Messaging Server)
 a1 login admin PASS
 a1 OK User logged in
 a2 proxyauth username
 a2 OK [REFERRAL IMAP://mailstore/] Completed
 a3 logout
* Zimbra (fails)
Code:
 telnet zimbra 143
 Trying x.x.x.x...
 Connected to zimbra
 Escape character is '^]'.
 * OK IMAP4 ready
 a1 login admin PASS
 a1 OK [CAPABILITY IMAP4rev1 ACL BINARY CATENATE CHILDREN CONDSTORE ENABLE ESEARCH ESORT I18NLEVEL=1 ID IDLE LIST-EXTENDED LIST-STATUS LITERAL+ LOGIN-REFERRALS MULTIAPPEND NAMESPACE QRESYNC QUOTA RIGHTS=ektx SASL-IR SEARCHRES SORT THREAD=ORDEREDSUBJECT UIDPLUS UNSELECT WITHIN XLIST] LOGIN completed
 a2 proxyauth username
 a2 BAD parse error: command not implemented

The AuthN appears to work fine above, but FWIW my Zimbra users are authenticating to the same remote LDAP server that Sun is. However, the Zimbra admin user is only locally defined.