MeFi Mail is a private messaging system for MetaFilter users and allows contact with users who may not have an email address in their profile. It is enabled by default but can be turned off in the profile editing area. Members can opt to have these messages sent directly to their profile email address.
There is a little envelope on the header bar of each page of MetaFilter, next to your username. The little envelope will turn orange when new messages are received. To check your Mefi Mail, click the envelope; it will take you to your Mefi Mail inbox.
To send someone Mefi Mail, go to their profile page by clicking their username. Members that have this option enabled will have a "send MeFi Mail" option visible on their profile page underneath their username that can be used to contact them.
Users can also opt to block MeFi Mail. Users who have blocked MeFi Mail can not send or receive MeFiMail and their profile page will say "MeFi Mail Disabled". To block Mefi Mail on a per-user basis, scroll to the bottom of a Mefi Mail from that user and click "Block this sender." If you have blocked a user and want to unblock them, you can do this from your contacts page at the bottom where it says "Manage blocked senders".
HTML cannot be used in MeFi Mail messages but URLs on a line by themselves will turn into hyperlinks. Banned or time-outed users cannot use or access MeFiMail.