This is a set of issues that are indirectly related to CalDAV; they are interoperability issues that manifest in CalDAV but their root cause is in Axigen's implementation.
We do not implement support for schedule status values.
RFC 6638 describes a mechanism that allows a CalDAV client to know the status of the iTIP delivery for a given attendee.
E.g. If the organizer makes a mistake when filling in the address of the attendee in the current implementation he/she can notice this mistake when he/she receives the NDR (non delivery report) for the iTIP message. If Axigen would implement schedule status values the CalDAV client would be informed of a delivery failure in the VCALENDAR item.
Notifications belonging to events accepted from Axigen WebMail remain in the Calendar.app INBOX.
At this point we have not found a way to tell Calendar.app to dismiss a notification that has already been processed outside the Calendar.app (in our case in WebMail).
Calendar.app cannot create events with custom monthly rules (repeats monthly on the 12th, 14th and 26th).
Axigen Calendar API is unable to parse the RRULE created by Calendar.app.
Calendar.app <-> WebMail/Outlook connector interoperability issue
Calendar.app has a different interpretation to RRULEs when interpreting yearly recurrent events.
It interprets the DTSTART as the first occurrence of the event, event if it does not match the date when the event repeats itself.
WebMail and Outlook however, display events only for those days that match the repeating rule.
CalDAV <-> WebMail interoperability issue
When a Axigen user is organizer in an event and the event is created by a CalDAV client (e.g. Calendar.app) editing the event via WebMail causes the event to be cancelled.
Calendar.app adds an ATTENDEE property for the organizer in the VCALENDAR.
When WebMail edits the VCALENDAR it removes the ATTENDEE property above.
The calendar API detects a change in the calendar object (an attendee has been removed) and sends a cancellation message to that attendee (in our case the organizer).
Axigen does not send attendee notifications for local folders that are not the default folder (Calendar)
Axigen current implementation - present in WebMail too.
Only events from the Calendar folder are taken into account when computing FreeBusy.
In Axigen no other local calendar folders implement FreeBusy.
Large calendar events (e.g. w/ attachments) can't be synchronised via CalDAV
Attempting to sync large events (e.g. uploading a calendar that's 12K or bigger -> eM Client attachment -> triggers error)