ST team... I know you are moving very fast towards Smartermail 17 and you want to add so many awesome features, but over all the versions, your implementation of CalDAV and CardDAV is very bad.
I have been trying to sync my calendar and contacts with Mac and Iphone and it doesnt work 100% well.
Specificallly with Mac native Calendar and with 3rd party products like BusyCal it fails to sync well.
Also with Mac native Contacts and 3rd party products like BusyContacts it also fails.
BUT on the other hand, all my icloud contacts and calendars sync like a charm!!!
It seems that need to have a development team look over the latest CalDAV and CardDAV RFCs and support sync collections and sync tokens on your CalDAV server for better, faster, smarter sync.
Please, please, IMPROVE these implementations before adding more stuff to Smartermail.