I would love to get the answer to this problem! I migrated from Exchange to SmarterMail and this is one of my biggest regrets due to the lack of outlook integration.

Vincent, does MAPI not work for you?

I am using IMAP vs MAPI
I am able to get the calendar into Outlook via CalDAV but not able to make it the default calendar within Outlook.

If you are using IMAP you will not get what you are looking for.

If you moved from Exchange to Smartmail while using IMAP will make you disappointed.

Smartermail MAPI is much more like Exchange then IMAP will every be.

Look into MAPI.  

Keneth:

We had mixed results with getting Outlook 2010 to accept an alternate calendar (any calendar, not just SM) as the default calendar; however, I do recall that it did work for some users in some builds.  You may want to take a look at the "Add to Outlook" feature available from the menu in the web interface.  We have not used it since v17 and MAPI coming available but I did just check and it is still there.  In v17 you may see an example by selecting the calendar then clicking on the ellipses.  It is located in a similar place in earlier versions.  When we did have success, we used the Add to Outlook method. How it works is well covered in earlier threads if you do a search.  We did have some success then syncing with smartphones using EAS and also with early Blackberries without a BES server when they were popular by using an EAS like 3rd party plugin but the Outlook part of it was the problem any time we had a problem.  It was better with 2013 but Microsoft gave up trying to fix both those versions I believe.  I do get that we are stuck trying to do the best we can for legacy clients though, no rant or soap boxing here.  On the bright side, with Outlook 2010 now well past EOL you shouldn't be plagued by updates resetting it to the default local calendar.