From the messagebus.com website:
Unfortunately, it does indeed affect the user experience; if you are using a mail reader such as mutt and a mail server like Postfix which haven't been trained to know what to do with their "X-Messagebus-Info" extensions, you simply cannot respond to the confirmation email, because you don't get HTML links, you get a proprietary "X-Messagebus" mime type.
Never mind that what should be direct communication between the Bittrex exchange and you is forcibly being sent through a third party (without your knowledge), for the express purpose of tracking you - it actually makes it impossible to register, impossible to confirm withdrawals and impossible to enable 2-Factor Authentication, unless one uses a web-based email service like Gmail or Yahoo.... Frustrating !
I was able to find some documentation for the family of mail extensions introduced by MessageBus, (names like "X-Messagebus", such as "X-Messagebus-Info", "X-Messagebus-Diagnostic") but it's just annoying to have to get your ISP to make major changes on their email infrastructure, just to be able to see a link to confirm your email address.
Bittrex, you have built a nice exchange; please don't let messagebus mangle the real links in your emails. At least demand that they keep an unchanged copy in what they regurgitate back out.