The "Configuring Inbound Email" document covers the technical/config steps, but doesn't actually walk through the application email setup stuff.
There's also a mention in "Spaces User Guide" that should probably be crosslinked.
We have a few different teams within our company that uses Phabricator for their own backlogs, and multiple teams would like to use the mail feature in Phabricator to receive new tasks to their specific backlog. One option we have is educate all of our users that they should write !project [teamname] and not type it wrong in all of their mails.
The other option we've identified would be to give them a mail address for each team, like firstname.lastname@example.org and email@example.com and that would then end up in Phabricator as an issue with the correct project attached. Now, we could attach the project to mail during inbound mail processing by appending !project to the body, but it is kind of fragile and it does not support encrypted emails either (I guess?). So, it would be really cool if you could control all of the !commands within mails with X-Phabricator--style headers as well as the in-body !commands. I would assume the !commands would take precedence if they contain overlapping or conflicting assignments.