I would like to have a button on my tool-bar to send an e-mail to a frequent correspondent. It would create a new e-mail with the "From:" filled with the correct account (I have several), the "To:" filled in with the recipient's e-mail address, and a particular closing for this correspondent. Alternatively, is there a way to create a template for an e-mail with all that filled in that I could attach to a short-cut key so that all I had to do was type the short-cut key and the e-mail would be ready for me to fill in.

In Eudora there used to be a way to do that and put a button on the toolbar. In fact, one could make multiple such buttons for different correspondents. That would be a nice add-on or feature for Thunderbird.