From blog.bitmex.com

Earlier today, some of our users received an email which contained the email addresses of other users in the ‘to’ field. We apologise for the concern this communication may have caused. This was the result of a software error which has now been addressed. BitMEX takes the privacy and security of our users very seriously. Rest assured that in this instance, beyond email addresses, no other personal data or account information have been disclosed and no further emails have been sent. The error which has caused this has been identified and fixed, ensuring our usual high standards of privacy are upheld. We are ... (full story)