Which email client supports sub-addresses and sending such mail to automatically generated folders?
For the email foo@foo.com, I want to be able to give someone foo+bar@foo.com and when they email me it automatically goes into a generated bar folder without any additional clicks or configuration from me.
However, it has a really nifty feature that makes this idea work much better, in my opinion. You can configure FastMail to use "virtual domains" that will reroute b@a.example.com to a+b@example.com. (To use this, you need a MX record for *.example.com, but that's fine.)
So, for example, I may sign up for a SpammyService with the email spammy@myemail.mydomain.com. Then all emails from SpammyService will be redirected to myemail+spammy@mydomain.com, which will then be filed in the appropriate folder.
This lets me (1) get around services that disallow plus signs in emails and (2) not let an automated system know that I'm auto-filing their emails (and potentially deleting them, etc.)
For the email foo@foo.com, I want to be able to give someone foo+bar@foo.com and when they email me it automatically goes into a generated bar folder without any additional clicks or configuration from me.