Hi Bas, this thread from the zarafa forums should help
You need to turn on the zarafa-gateway service as access for IMAP is via the Zarafa MAPI. I've not explained this in the howto as it conflicts with existing ClearOS mail service ports. However to do this so it doesn't you will need to edit /etc/zarafa/gateway.cfg and change them to other values (default is 143 for IMAP, and 110 for POP). Some examples simply include adding an 8 to the front, ie. 8143 for imap...Then run the gateway service, 'service zarafa-gateway start'