Stock sendmail will try to lookup the sender domain, if the sender domain has an A record for @, it will use that instead of the MX to determine if the mail is coming from a legitimate source or not. It was a derpy spam prevention method started years ago, I doubt its been removed, but I also don't use sendmail a ton anymore so its possible its finally gone.
Also, a fair amount of the world is used to the convention of www.something.whatever so you'll have people complaining your site is down if you don't have www. Firefox will also sometimes prepend www to be "helpful" as well if there's some other typo in the URL (not a fan of that).
