You know it worked in the past, you don't know that every other piece of the system has not changed. Maybe your mailserver changed settings, maybe your hosting provider blocked a port, there are any number of possibilities as to what the cause of the issue might be. If you can roll back to 3.6.x and it works, you've likely identified the cause of the issue. But simply stating "it used to work, it doesn't now", is not the same thing.
@Cyberster: Did you find a solution? If you tried with a previous WP version and it works, but does not work with 3.7.1, please let me know which version works and I'll look into it further. I also recommend contacting your mail provider to verify if their setup has changed at all.