When using EWS Managed API to send Email via Exchange Server 2007. I noticed that MAPI clients like MS Outlook display all custom headers. But when I use POP3/IMAP clients like MS Outlook Express. I have noticed that these custom headers do not display in the message opened from MS Outlook Express. Is there a setting in Exchange Server 2007 that we can set to make these custom headers propagate and be received by a POP/IMAP client? Also why do custom headers in example below display up in lower case in MAPI clients like MS Outlook? But surprisingly if we use SMTPClient class to send email then these headers display as sent with Case Sensitive letters. eg. Header. Example of Headers received by a MAPI client like MS Outlook via Exchange Server 2007 Received: from EXMAILVS1.blabla.com ([192.168.191.136]) by cashtp02.blabla.com ([XXX.XXX.XX.XXX]) with mapi; Mon, 20 Dec 2010 12:17:05 -0800 Content-Type: application/ms-tnef; name="winmail.dat" Content-Transfer-Encoding: binary From: asfsdf <[email protected]> To: asdsdf <[email protected]> Date: Mon, 20 Dec 2010 12:17:04 -0800 Subject: Please send me this header Thread-Topic: Please send me this header Thread-Index: AQHLoILek7g5cFgHQU6lHHfiKkdUMg== Message-ID: <[email protected]> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-Exchange-Organization-SCL: -1 X-MS-TNEF-Correlator: <[email protected]> customheader1: hello ali customheader2: hello Jace MIME-Version: 1.0