No way that I know of in Pegasus Mail to examine and edit the HTML in a message being composed.  If you need that sort of control in may be better to compose and edit it in your favourite HTML editor, and then import it into the message area using the right-click 'Import HTML file' option.

I'd be grateful if someone could tell me whether these are possible, and if so how: 

a) Seeing the HTML source of a received message?

b) Seeing and editing the source of an outgoing message?

OK - found reference to the "raw" tab in a recent messsage, so incoming is take care of.  Still would apreciate help on the outgoing.