I am testing zimbra desktop with our imap server. when it connects to imap server it shows no message at all if the server returns FETCH result using INTERNALDATE format like the following:
1. INTERNALDATE "Sat, 12 May 2012 11:56:21 +0800"
however zimbra works fine if the date format is like the following
2. INTERNALDATE "12-May-2012 11:56:21 +0800"
I am not sure which is correct but if we use format #2 that zimbra accepts, some other platform/clients don't show any message at all.
I believed that if zimbra desktop accepts both formats, it will be compatible with more imap servers.
Evo Mail Server (no english content at this moment.)
If you want to test with it, there is english version of the Imap mail server downloadable at here https://www.evo-mailserver.com.tw/download/evoms_en.msi