1) So use use the Flex SDK and your favorite editor. Or, if on Windows, flashdevelop. Or, if you like Eclipse and have some cash to spare, Flex Builder.
2) Your opinion. Assuming it was done well, a Flash/Flex based mail client could rock. As an added bonus, done right, it could also be a client side mail client (using AIR).
Also, if you're using Flex controls, reskinning isn't that hard.
3) Again, your opinion. Without knowing what you mean, I can only assume you mean the whole Flash Authoring Tool (damn Adobe for overloading Flash to mean the IDE, the VM, the framework, and the technology)/time line thing, you don't need to use it. There are loads of options. See #1, above.
2) Your opinion. Assuming it was done well, a Flash/Flex based mail client could rock. As an added bonus, done right, it could also be a client side mail client (using AIR).
Also, if you're using Flex controls, reskinning isn't that hard.
3) Again, your opinion. Without knowing what you mean, I can only assume you mean the whole Flash Authoring Tool (damn Adobe for overloading Flash to mean the IDE, the VM, the framework, and the technology)/time line thing, you don't need to use it. There are loads of options. See #1, above.