I recently had a discussion with Exchange administrators from a couple of major Universities to decide whether it would be possible for someone to use their Exchange mailbox in a "calendar only" capacity. The end user would like to maintain his or her own Email but they would like to still take advantage of group calendaring. The short answer is you cannot disable the other components of a mailbox and there is no "Calendar Only" check box. The process of scheduling and responding to meeting requests is driven by sending and receiving Email messages.
So, here's one approach. Given that calendaring is very reliant on Email and we want to keep our routing as simple as possible, I would start by routing the user's Email through Exchange. Obviously there may be political issues as to why a user might not want to route through someone else's mail system but we will assume this is provided on an "opt-in" basis. In this scenario, routing mail through Exchange prevents a number of complications. Assuming Exchange has a recipient policy for "name.edu" Exchange will attempt to locally deliver any messages sent to an alias within this domain. Therefore if a user taking full advantage of Exchange sends a message to a "calendar-only" user, the message will not leave the server unless a forwarding mechanism is in place.
We need to implement two server-side rules for each mailbox from within Outlook. Unfortunately you must use the full Outlook client since OWA does not offer options to take action based on type. This can be completed by the user or by an admin with temporary access to the user's mailbox.
The first rule redirects any message sent to the user to a mailbox on an outside server. You can eliminate this step by administratively setting the ms-Exch-Deliver-And-Redirect and ms-Exch-Alt-Recipient attributes in Active Directory.
Then enter a second rule:
Make sure the first rule is listed first and the second rule is listed next. The rules are applied in order and you don't want to delete messages before forwarding them! There is potential for the user to misconfigure these rules but proper guidance and web based documentation should help lead to fewer user errors. It is possible to configure these rules centrally for each user, although it is not a simple process and does require developer experience. See support article 251125.
Now you have setup an environment where mail is always forwarded to the desired mailbox on another system but anything calendar related will still be available in Exchange for scheduling. The messages will also be delivered to the user's remote mailbox so he or she will know when a new item needs attention.
One final note. The deleted messages will end up in the Deleted Items folder and will not automatically be emptied. The Exchange Server administrator may choose to setup mailbox maintenance rules to empty the Deleted Items for these users on a regular basis, for example after 7 days.
This is one possible approach. University of Iowa has taken a similar approach documented on their ITS site. If others have come up with methods such as event sinks or customized delivery of messages by type, I would be very interested to learn more and leverage this site to share details.