Windows Live Mail is not Outlook. It's an entirely different mail app, so thanks for clarifying this.
Now, does this happen to any HTML message? Only when replying? Only when replying to certain messages?
Can you and your co-worked try a *new* message in AquaMail, not a reply to an incoming message?
Now, I suppose it's pretty obvious that a mail app should not crash, no matter what's in the message (even if it has something invalid, and that's not necessarily the case), even if it's a mail app from a large corporation.