I'd tell them to fuck off and abandon the account myself, or I'd charge them for my time in getting the info together not that I could be arsed to get shit notarised, but I doubt it's massively out of the ordinary since they now have fiat dealings. They probably spring it on random customers so they can show the authorities that they're being proactive when they come snooping.
Other exchanges do the same thing every now and then. I've only ever had a bank do it once which says something, and I made it all up and they went away.
Are there any funds in there right now?
I removed all funds as soon as I saw the mail, I've read about a few people having the same thing (not recently though) and they got their funds frozen despite complying. I've got little interest in getting into a never-ending struggle to maintain control over my own funds.
Asking for sources fo funds is a different story though. Is there any reason that you think why or could you possibly be linked to someone they think to suspect?
I moved & sold most of my funds several months ago, might have triggered something (eg. they are forced by -fill in favourite intelligence agency- to share transactions to or away from their platform adding up to a certain amount).
Little doubt that they had to make a proverbial pact with the devil to enable fiat trading.
I used to be quite fond of Bittrex and Bill Shihara but this has hastily changed my opinion.
What really disturbs me is the fact that they use personalised links in the mail so they can easily track which users are complaining about it online & copy paste the mail.