The custom MIME property will not be automatically promoted to a named MAPI property from a MIME header. Exchange used to that, but not anymore - since you can have at most 0xFFFF unique named properties, this is a nice denial of service channel: just send a message with a few thousand x-headers to a mailbox, and you will permanently toast it.
If you see the custom x-header in PR_TRANPORT_MESSAGE_HEADERS
(use OutlookSpy - I am its author - and look at the message to see that and all other MAPI properties - click IMessage button), you can read the MIME headers and programmatically extract your custom header. PR_TRANPORT_MESSAGE_HEADERS
can be accessed using MailItem.PropertyAccessor
, its DASL name is http://schemas.microsoft.com/mapi/proptag/0x007D001F
.