Have you tried eReceipts in Mail?

I've looked into it, but it's a cumbersome workaround and involves email attachments; what we want is a way to easily thank donors by email, without using an attachment, and easily as writing them a traditional letter. It's as if Outlook integration has only been partially implemented in RE, and it's infuriating.

I would add that email needs to be an "address type" in all mail processing, so that if someone has a valid email address then the "letter" can go out as an email, if they don't then it gets generated as a standard paper letter.