not sure what you are saying about the quick extension but to answer the first part of your post.. yes, it's a pretty secure email client.. i know of a lot of corporations (including mine) that use it.
(i'm a field rep and i visit a lot of tech companies)
Generally Notes is secure but like anything there are minor bugs and such that do happen. I would suggest going to their Updates site and getting necessary patches and such. Their Developer Domain has all the M/RU files.
In addition, it is as secure as the OS it's on. So if its NT make sure its up to date (but avoid Service Pack 6 -- use SP 6a instead).