I battled with IBM that I wanted all internal emails kept as rich text since buttons/stored forms got converted to html and where lost.  As far as I know the only way you can trick it out is by having the internet address repeated as the first value in the full name field and once the use presses f9 it will resolve to the first full name.   This suggestion might cause other issues,

You might want to log a ticket with IBM to see if there is a way of forcing it on the server (hidden ini)

I battled with IBM that I wanted all internal emails kept as rich text since buttons/stored forms got converted to html and where lost.  As far as I know the only way you can trick it out is by having the internet address repeated as the first value in the full name field and once the use presses f9 it will resolve to the first full name.   This suggestion might cause other issues,

You might want to log a ticket with IBM to see if there is a way of forcing it on the server (hidden ini)

I tried that too and it is still only sends in Rich Text.  I also opened a ticket with IBM and got back several emails with tech notes on how to change client preferences for out bound mail to MIME, how to create an HTML signature, and how to check the Keep in Senders format option.  Nothing of value for internal mail.  I wouldn't think it would be so difficult to send email as HTML, or as IBM calls it Internet format.

So far the only way I can force the notes client to send HTML is to manually type an address in as <address@domain.com>.