Filtered mail is like a mail junkbox. The purpose of filtered mail is to give you control over who can send you a private message.
For example, if you only want to be contacted by women over the age of 35, you can set this up by doing the following:
1. Go to account settings (My account) and clicking on the "Edit Profile" section.
2. Scroll down to the "I'm happy to be contacted by" section.
3. Select the genders that you're happy be contacted by. By default, all genders are selected.
4. Select the age range. By default, there is a wide age range.
5. Scroll down and click on the "Update" button.
Anybody contacting you who is not the specified gender or within the age range that you have selected can still contact you, but their messages will go into the "Filtered" inbox instead of the main inbox.
When someone sends you a mail that is filtered, you will not get the usual blue dot on the envelope. Additionally, you will not receive an email notification telling you able the mail.
How to make exceptions
If you want to be notified when certain people contact you, but those people would be caught by your filter, you can add those people to your favourites. Anybody on your favourites list will bypass your mail filter settings.