Connection to account lost when synchronizing appointment to Outlook

We're using CRM 2011 and Outlook 2010. Normally we put an account in the "Set regarding" field when creating an appointment in CRM. This synchronizes to the user's Outlook calendar, and all is good. However if we try to put a campaign activity in "Set regarding" and set the account as a mandatory part of the appointment, it seems OK at first glance. But if the customer don't have an e-mail adress registered (happens often with new/potential customers), then the appointment disappears from the customers activities after a while (approx. 30 minutes). This happens ONLY when the customer don't have an e-mail regsitered. I guess this have something to do with Outlook using the e-mail adress as link to CRM, but why is the appointment lost in CRM?? It still shows in the Outlook calendar...