For some backup purposes, we are actually working, for each account as this :
Zimbra : account@domain.com
Exchange : account@domain.com

I made the imapsync from Exchange to Zimbra and it worked well. Everymail received on Exchange is copied to Zimbra thanks to aliases. The only problem is that read/unread messages are not synchronized. I tried with imapsync but it ended in a big fail as every message is duplicated, problem with dates.

Has someone any clue on this ?

By the way, I'm searching to synchronize calendar as well, I made it thanks to iCal4OL but if someone knows a better Outlook plugin to synchronize with Zimbra's ical, I'd be interested

Thank you