The default mail client on OS X is Mail.app. In recent versions it does it’s best to bundle threads of email together to make it easier for you to keep track of conversations via email – they appear in the list of messages as a single entry with a badge showing the number of messages in that thread. There are standard ways to track mail threads, but they sometimes get broken by mailing list software, so Mail.app also bundles together messages with an identical sender and Subject line.
That has an unexpected side effect, when it comes to email marketing.
That little “4″ badge on the right tells me that this is the fourth time Marriott have sent me this same email (over a period of several months) and there’s really no need for me to open it and read it again.