Happen to have a calendar that is email enabled on SharePoint but calendar items aren’t being created from an email?
Check your ULS logs on your SharePoint server that and see if you have something similar to this error below:
An error occurred while processing the incoming e-mail file C:\Inetpub\mailroot\Drop\86d3c93401cab0ba0000002b.eml. The error was: Value cannot be null. Parameter name: stream.
This is most likely related to using an Exchange contact that forwards to your SharePoint calendars email address. Change Use MAPI rich text format to Never.