While going through my inbox, I notice a mailing list that I have no memory of subscribing too... For a good reason: I probably never subscribed to it.
How do I know? I use specific email addresses for commercial services, making the origin of an email easy to trace. Of course, I always opt-out of mailing lists while checking out / signing up for services.
Then, if I receive an email to this email address from a different sender, it's probably because the initial service provider sold the email address to the sender. In this case, my "makemytrip@_domainname_.com" went to Payback for some reasons. Maybe I forgot to un-check some (hidden) checkboxes. Maybe I choice was simply not respected. I now unsubscribed from this mailing list, and hope no other will follow.