I just recently set up Google Apps (Standard) on my domain, and I have a catch-all e-mail address on it that I pull e-mail from locally, so I can easily receive e-mail to
firstname.lastname@example.org. Now I want it to work in reverse - I want to be able to send from any arbitrary address on my domain, by just including a
From: email@example.com header with my message. Unfortunately Gmail will rewrite the header to use whichever e-mail address I authenticated as.
firstname.lastname@example.org doesn't actually exist (the incoming e-mails to it are caught by the catch-all, so there's no need to make an actual
foo account), I can't authenticate as it; I have to authenticate as my catch-all address, and then the e-mail shows up as sent from that address regardless of the
From header. Gmail does support adding additional valid 'From' addresses (in Settings -> Accounts) but I don't want to do that every time I have a new
email@example.com I want to send from.
Is there a way to tell Gmail/Google Apps "I control all e-mail from
example.com, as long as the 'From' address is on that domain just leave it alone"?
As far as I know it's just not yet possible to "delegate whole domain" on gmail.
Better multiple identity and signature management. Set up multiple "identities" in Thunderbird with email address-specific signatures, which you can't do in web-based Gmail. Hit the "Manage Identities" button in your Account Preferences dialog. The various identities you choose will be available as a dropdown in the From: field in new messages, just like in web-based Gmail. You can also create and automatically attach a vCard to your outgoing messages on a per-identity basis with T-bird, and choose to compose your messages as HTML or plain text per identity, too.
About your other unintended question that I accidentally took as the actual question before:
Is there a way to tell Gmail/Google Apps "I control all e-mail from example.com, as long as the from address is on that domain just leave it alone"?
Yes there is. Just add a domain alias!
It will behave just like you expect: it points everything from the alias to the main and makes all of them work seamlessly together.
No, there is no way to do this at this time.
Your only option using Gmail's interface is to add the address as a "Send mail as" address.