Use of the principal field is usually done in lotusscript but there is no reason why it cannot be done in @formula language.
In fact it is easier in formula language.
The trick is to use @mailsend by itself ie use mailsend without any parameters, not even brackets.
Create a form with Sendto, subject, body and principal fields.
copyto and blindcopyto and even subject are optional, but you really need sendto for mailsend to work without any parameters.
In a button just put @mailsend.
NB: You cannot use the [IncludeDocLink]etc options with Mailsend unfortunately, so if you need to use this you must do it in lotusscript.
NB: If you change your Location document's internet address field to the value in Principal as well, you can completely spoof hotmail.com and mail.com etc. Outlook and Lotus Notes are not as easily spoofed though thankfully.
I have added this to my Utilities database which will be available on http://www.Notestemplates.com shortly.
-Adam