Hi,
Please open the user's Outlook and find the contact item for you, then follow the steps below to check if the user has specified a format for you:
1. Open an Outlook contact.
2. In the Email box, double-click the email address.
3. Click View more options for interacting with this person.
4. Click Outlook Properties.
5. In the Internet Format list, check if it is set to
Send Plain Text only. By default, Let Outlook decide the best sending format is selected, which usually is
HTML format.
For more information, please refer to the link below and see the "Change the message format for all messages sent to a specified recipient" section:
http://office.microsoft.com/en-in/outlook-help/change-the-message-format-to-html-rich-text-or-plain-text-HA101992313.aspx
Best Regards,
Steve Fan
Forum Support