Outlook 2007 Ignoring Mail Format Configured for Contact

I posted this question in microsoft.public.office.misc before I find this
group... So I am posting again.

I need to send emails to a particular internet email address (i.e., not in
exchange) in Plain Text. I attempted to configure this in the following ways:

Attempt 1:
1) Open Contact window
2) Double click Email... field under Internet section
3) Set Internet format to "Send Plain Text only"

Attempt 2:
1) Open Contact window
2) Right click Email... field under Internet section
3) Select Send Option...
4) Check "I want to specify the format..."
5) Select "MIME" - "Plain Text"

Attempt 3:
1) Same as steps 1-4 in Attempt 2
2) Select "Plain Text/UUEncode"

However, all attempts failed. Outlook still sends the email in HTML after
all this settings. This is not only when I reply email from that address, but
also when I send new email - even if I click the 'E-mail' button on the
Contact window.

Besides, is it possible to add a 'Reply in Text' button in the UI
(preferably both in the mailbox view & mail view)? I've tried VBA, but it is
disabled by default. Besides, it seems not an easy job to add a button to the
ribbon.

Telling people to go to the Options ribbon and select Plain Text impossible,
they will ignore such instruction.

I also know it is possible to configure mail format globally from the Tools
- Options - Mail Format. But this will affect all mails, while I want to
configure this for a specific recipient only.

Any suggestion?

Thanks

Outlook version: 12.0.6514.5000