At work, we use a Kerio mailserver. To Windows Mobile and Windows Phone devices it looks like an Exchange Server, so you can add your account as an Outlook/Exchange account. Well, in theory. I tried this before, but wasn't able to add the account. I could create a POP3 account, but that left me with e-mail only, not my calendar.
Yesterday I tried again. With a colleague looking at the Kerio logging (thanks Sander!), we found why creating the Outlook/Exchange account on the phone was not working.
When creating the account you need to provide several settings, amongst which is the Domain. I simply assumed this should be the domain of the account (as in domain\username), like when connecting to a real Exchange server. Not so with Kerio. You need to provide the full domain of your e-mail address. So in the case of firstname.lastname@example.org, the Domain setting should be domain.com.