There are some circumstances where you might want to view an email message in a web browser. For example, you might receive a message that doesn’t render well in Microsoft Outlook. Or perhaps you want to print a message using a more controlled interface; for example, you might want to print a single page of an email message; see my tip "Printing Only the First Page of an Outlook 2007 Email." Outlook 2010 provides a simple mechanism for viewing email messages in a browser.
Learn more recent information from, "Exchange 2013 Site Mailboxes - a new beginning for collaboration?"
When Outlook identifies that a message might have some rendering issues, it includes advice in the MailTips section of the message: “If there are problems with how this message is displayed, click here to view it in a web browser.” Figure 1 shows this text highlighted in orange and preceded by a lowercase “i” (for "information") within a blue circle. Clicking this banner reveals a context menu, also shown in Figure 1, which includes the option to View in Browser. You can also find the View in Browser option in the Move section of the Ribbon of an opened message. Select Actions, View in Browser. This option opens Internet Explorer (IE). It won’t open your default browser if you use a default browser other than IE. It’s an IE only feature.
Outlook saves a copy of your message as a web archive (a Microsoft proprietary format) with the extension .mht in a temporary files folder. For example: file://localhost/C:/Users/<username>/AppData/Local/Microsoft/Windows/Temporary%20Internet%20Files/Content.Outlook/WW7HRH1C/email%20(3).mht. By default, files with the .mht extension are associated with IE. It’s this file association that determines which application is opened. You can change this file association within Windows if you want another browser to try to view messages, but not all can render .mht files. Firefox and Chrome can’t do it by default; however, Opera can render .mht files, as shown in Figure 2.
