Date refers to "origination date" or "creation date" or "submit date"
ReceivedDate refers to "arrival date"
The origination date specifies the date and time at which the creator of the message indicated that the message was complete and ready to enter the mail delivery system.
The arrival date specifies the date and time at which the mail delivery system received the message for processing (e.g. message was written into mailbox).
However, value of the
MailMessage.Date property is taken from the 'Date' MIME header. If it is not specified, the value specified by the first 'Received' MIME header is used instead. In this case,
ReceivedDate have the same value (which can be null).