Well, this is interesting. I just can't think of what we (SOGo community) can do to improve this our end. Short of removing monthly recurring events (LOL) there's not much we can do. There's no way to tell if the recipient is using Outlook or otherwise.

This is purely a Microsoft and their customers issue.

In the meantime, it may be best if SOGo users be careful when inviting to recurring events. Perhaps only use this feature internally, and avoid using Outlook in their organisation.

I think this is related to bug 0003364:

http://www.sogo.nu/bugs/view.php?id=3364

Reopen if necessary, it works well now from my testing.