There doesn't seem to be such a site yet. There certainly isn't one with critical mass, like GL&U has, otherwise it would (almost per definition) be easy to find on google. There might conceivably be a tiny forgotten one hidden away somewhere on the internet, but if it's so small that google searches won't pick it up, it's unlikely to be of any use to you.
I also see a few issues that really make me doubt whether it's a viable idea, too:
Confidentiality: Are people really going to want their work emails out there for all the world to see?
Return on time invested: What would native speakers get out of correcting your email? On stack exchange sites, experts are rewarded for their efforts by being able to get their own questions answered in turn. What would a fix-my-writing-for-me site offer them?
On stack exchange sites, answering a question helps more than one person - you're creating value on the internet, and helping everyone else that's going to have that issue in the future. Fixing your specific email helps only you. Are people going to feel like that's a valuable use of their time?
Some of the least popular questions on stackoverflow are the ones that are just a ton of buggy code with "Here code, you fix," because it feels like you're dumping a bunch of work on people with absolutely no indication that you've done any work yourself. People want to help, but they also want to feel like you're at least trying to pull your weight, not just taking advantage of their generosity.
I can get quick answers here but only if I spend the time to formulate them into a question
but the time and effort you spent turning your issue into an interesting question is the very thing that makes people want to help you out.
The site you want would allow you to get a ton of free labor out of people with absolutely no effort on your part, and to be honest, I think there are good reasons no such thing exists.
Your best bet is probably to find a few native speakers who are willing to correct your email for you in exchange for you correcting their English email for them.