No, there's no legal problem in being a user's mail client. The e-mail belongs to the USER, not to the hosting provider, and the user can access it with whatever compatible client they like.
That said, I'm pretty sure I wouldn't even consider using a web-based encryption service that wasn't from my hosting provider. Using a third-party service to read my e-mail means that not only do I have to worry about my provider leaking my information, I have to worry about this other service leaking my information.
I can't think of any way that anyone would appear trustworthy enough for me to double my exposure like that, when I can run an encrypting e-mail client on my desktop or my phone and at least have some control.