We have a similar issue with just a handful of users, all upper execs, as luck would have it. The invitation never shows in the Inbox or Messages folder, but after about 20 min. shows up on the Outlook and Blackberry calendar as tentative, before disappearing a little later on.
We're running Exchange 2003, SP2, with the CDO.dll versions 6.5.7638.2 and mapi32.dll versions 6.5.7638.2 matching between the Exchange server and BES.
The handhelds are 8700c w/ v220.127.116.11 OS
Our BES is currently 18.104.22.168. The concerned users' clients are Outlook 2003 SP1.
Most perplexing. Any help would be much appreciated