The Winmail.dat attachment includes formatting, attachments, and Outlook-specific features such as meeting requests. Typically, this attachment is named Winmail.dat. A TNEF message contains a plain text version of the message and an attachment that packages the original formatted version of the message. TNEF is a Microsoft-specific format for transmitting formatted email messages. Transport Neutral Encapsulation Format (TNEF) The following table describes the message transmission formats that Exchange Online uses to send email messages to external recipients. Message transmission formats for mail sent to external recipients Please note that RTF messages that are encrypted with S/MIME will face limitations and may be prone to face conversion issues (like journaling delivery).Ĭan read messages formatted in RTF, but can't format or send this format Only Outlook, Outlook on the web, and a few other MAPI email clients understand RTF messages. RTF supports text formatting and other graphical elements. The message can't contain different fonts or other text formatting.Īn HTML message supports text formatting, background images, tables, bullet points, and other graphical elements. The following list describes the three message formats available in Exchange Online, and shows which ones are available in Outlook and Outlook on the web: FormatĪ plain text message uses only US-ASCII text as described in RFC 2822. Message transmission format can be set by an admin per domain or per recipient, and users can also specify message transmission format. Only HTML and plain text can be sent using MIME. All three message formats can be sent using TNEF. Exchange can send messages to other domains by using Multipurpose Internet Mail Extensions (MIME) or Transport Neutral Encapsulation Format (TNEF). Message transmission: This means how the message is actually sent to the other email system. In Outlook on the web (formerly known as Outlook Web App) they have a choice between plain text and HTML.
In Outlook, they have a choice between plain text, HTML, and rich-text format. Message format: When a user creates a message, they can choose the message format in which to author the message. There are two types of settings you can use: For example, you can prevent recipients from receiving a winmail.dat attachment. If specific recipients have trouble reading messages sent from your organization, you can adjust the settings for individual users, or for all users on a specific domain. There are settings in Outlook, Outlook on the web, and Exchange Online that control the format of email messages and how they are sent to people on other domains.