You haven't mentioned passwords. Use a good one. [http://www.explainxkcd.com/wiki/index.php/936:_Password_Strength] EDIT: I'm not a security expert. I'm in the same boat with Bruce Schneier as far as my knowledge is concerned. I suppose someone with better mathematical skills could run a few calculations.
Also of note is that while you may tell Thunderbird to delete an email, it might actually still be stored on the email server itself so you might have to manually delete it from there.
Lastly you could also promote some GPG in there, although that's a different can of worms.
You haven't mentioned passwords.
He also hasn't mentioned that you should exercise regularly, eat vegetables, invest in index funds, and not get addicted to heroin. Why should there be anything about passwords in an article advising people on how to get accumulated cruft out of their email inboxes?
This will be brief.
Inbox zero is a valuable thing to maintain. Roughly promoted around the web as having an empty inbox.
An email inbox collects a few things: