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PidLidUseTnef Canonical Property
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Specifies whether Transport Neutral Encapsulation Format (TNEF) should be included on a message when that message is converted from MAPI to Multipurpose Internet Mail Extensions (MIME) or Simple Mail Transfer Protocol (SMTP) format.
Long ID (LID):
This property specifies whether TNEF should be included on a message when that message is converted from TNEF to MIME or SMTP format. This property may be absent, and if so, TNEF must not be included on the message.
This property only applies when the message is sent from a POP3/SMTP mail account and does not apply when the message is sent by other providers, such as Microsoft Exchange Server.
Under certain circumstances, such as when voting buttons are enabled or an OLE embedded object is attached to a message, Outlook can set this property to force the use of TNEF.
The PR_MSG_EDITOR_FORMAT (PidTagMessageEditorFormat) property can be used to enforce only plain text, and not TNEF, when sending a message. Because PidLidUseTNEF overrides the setting in PR_MSG_EDITOR_FORMAT, an application that wants to force plain text on an outgoing message should also look for PidLidUseTNEF and reset it to FALSE. Additionally, the add-in must remove the message features that required TNEF to avoid unusable attachments on the message that is finally sent.
Use the CCSF_USE_TNEF flag when calling IConverterSession::MAPIToMIMEStm to convert an outgoing MAPI message to a MIME stream can also enforce TNEF. This applies even if PidLidUseTNEF is not set.