The message format for an Internet email address contact can be specified. For example, if you want all messages that are sent to melissa@contoso.com to be in plain text format, you can add that information to your Outlook contact for Melissa.
Open an Outlook contact.
In the Email box, double-click the email address.
Click View more options for interacting with this person.
Click Outlook Properties.
In the Internet Format list, click Send Plain Text only or Send using Outlook Rich Text format. By default, Let Outlook decide the best sending format is selected, which usually is HTML format.
Greeting,
Peter