Whatever happened to the Penelope project?
The link you gave is not too informative, but a Search about Penelope yielded some information, especially, old faithful Wikipedia
Pnelope is/was not integral part of Thunderbird, at any time, but rather an extention (add-on).
I have not been exposed to Office, at any time, and, when I got my first connection to the Internet, I was set up with Netscape, and consequently Mozilla. Thunderbird comes along for the ride, but I am doing fine with Seamonkey. At one time Eudora was my favorite program.
By the way the add-on could not be installed, because it it was not compatible with that version. If I cared, I would have tried to go around it, but, I, honestly, have wasted so much time today, that I don't care.
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:12.0) Gecko/20120429 Firefox/12.0 SeaMonkey/2.9.1