Did you deposit this BTC just recently or did you hold it for a long time on Bitmex? is the account was created a long time ago and verified?
Actually, it's a huge amount of money they might think that you are suspicious. If you deposited such a huge amount without the account being verified and just recently verify it because you have a problem withdrawing the BTC then I believe this would be problematic. It could take months or a year before they will let you allow to withdraw based on what I heard from other people experienced.
Keep contacting the support and try to ask them what you should do to solve this case.
Or you can spread this issue on their social media accounts if they can't able to answer you on emails.
Check this list of contacts here https://support.bitmex.com/en/support/solutions/articles/13000067278-official-bitmex-communication-channels
The accounts were created a long time ago. I am trading on these accounts.
That problem is not about depositing huge amounts of BTC or with AML or something similar.
I try to communicate with support, but they avoid answering. I try everything. Now I only want to find some people with the same experience, because I think it is not only my problem. I think that they try to "scam" some other people.