Received Email change to plain text automatically

If the receipt set the following setting in his/her outlook,

 

To turn on the Read all standard mail in plain text option in Outlook 2003, follow these steps:

1. Start Outlook 2003.

2. On the Tools menu, click Options.

3. On the Preferences tab, in the E-mail area, click E-mail Options.

4. In the Message handling area, click to select the Read all standard mail in plain text check box.
   
   Note By default, the Read all standard mail in plain text option is turned off.

To turn on the Read all standard mail in plain text option in Outlook 2007, follow these steps:

1. Start Outlook 2007.

2. On the Tools menu, click Trust Center, and then click E-mail Security.

3. Under Read as Plain Text, click to select the Read all standard mail in plain text check box.

4. To include messages that are signed with a digital signature, click to select the Read all digitally signed mail in plain text check box.

When the Read all standard mail in plain text option is turned on, you receive the following notification on the InfoBar at the top of the e-mail message:

This message was converted to plain text.

Note If you decide to view the plain text message in its original format, click the InfoBar, and then select Display as HTML or Display as Rich Text.

 

For more information:

How to view all e-mail messages in plain text format

http://support.microsoft.com/?scid=kb%3Ben-us%3B831607&x=5&y=19

Received Email change to plain text automatically