This is an email sent into the forum. Each category an have a separate incoming email address. The default email is forum-lobby@commonscomputer.com and shows up in the lobby.
You must already have a forum account to email into the lobby. You can add multiple email addresses to your account so you can send from any of them.
Turns out, when creating the inbox, you need to check the box that says “Enable POP3 Access”.
– Boris