Stuffy Inbox?

Is your e-mail inbox stuffed to the max? Here’s my theory:

I keep my inbox, nearly always, at less than 10 and often less than 5.
Some people don’t care; my son, for instance, uses his inbox to store the mail addresses of all those he might write to. Others, though, try to keep it down and here’s my question today:
What keeps you from deleting mails? What are your principles?