Yes that is the proper e-mail address where I've gotten replies. They USUALLY reply but once they did not reply at all.
I'm not sure, but certified mail might only be available to US residents, at the US Post Office. It is a regular letter you send but you get proof that it is sent by you and you get proof that they have received it. Usually anything like DHL, UPS, Fed Ex have this type of service too I believe. That's all you want is proof it was delivered, something like signature confirmation works too. Then you will KNOW they have the letter. Include in your letter that you are giving them 2 weeks to respond because you have tried numerous times to contact them via e-mail. Include your e-mail address in case they can respond by that method.
Tell them exactly what you want in the letter as well. This is just my suggestion. After this next step is to get a lawyer in the state where CoinMKT is located, if you're talking about a lot of money. You can tell them in the letter that is your next step, even though you don't want to have to.
Thanks Jcw188 for the very helpful information!