We sometimes send calender data or tasks using iCalendar (ICS) via eMail as specified in RFC 5545 Show archive.org snapshot .

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Recently, a customer noticed that Outlook automatically moved eMails containing such ICS data to deleted items folder and shows the events as tentative on calendar.
This problem is reported on TechNet Show archive.org snapshot , for example.

It seems the problem occurs whenever the organizer of an event is identical to the recipient of the email.
Typically, ICS formatted events contain a line such as

ORGANIZER;RSVP=true;CN="Jenna, Lina":mailto:jenna@example.com

If an iCalendar event is delivered to jenna@example.com, Outlook seems to delete the Mail immediately.
You can change the organizer to something different (noreply@example.com) and Outlook will keep the eMail.

Keep in mind that if you requested a response to the iCalendar eMail (by seeting RSVP=true as shown above) replies will be sent to noreply accordingly.