This parameter specifies the version of the MIME protocol that was used by the sender.

Content-Type: multipart/mixed;
This is an additional MIME header. It tells MIME-compliant email programs about the type of content to expect in the message.

X-Mailer: Microsoft Office Outlook, Build 12.0.4210
This information indicates that the message was sent by using Microsoft Office Outlook with a build version of 12.0.4210.

X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165
This entry indicates the email software (MIME OLE software) used by the sender.

Thread-Index: AcON3CInEwkfLOQsQGeK8VCv3M+ipA==
This header is used to associate multiple messages with a similar thread. For example, in Outlook, the conversation view uses this information to find messages from the same conversation thread.

Return-Path: [email protected]
This entry specifies how to reach the message sender.

Message-ID: <[email protected]>
The message has been assigned this number by mail.litware.com for identification purposes. This ID will always be associated with the message.

X-OriginalArrivalTime: 12 Dec 2017 21:38:50.0145 (UTC)
This is a time stamp placed on the message when it first passes through a server running Microsoft Exchange.