If your WordPress generated emails are not going through, your messages may end up in the user’s spam filter. In many cases the mail from some servers is blacklisted before it even gets to their spam folder. They can be completely lost in cyber space!
WordPress generated emails are things like contact form notifications, comment notifications, and passwords sent to new users.
WP Mail SMTP. This plugin hasn’t been updated in a few years but it still works fine in WP 3.6 (Oct 2013).
The port to use for a gmail address is 465, the server is smtp.gmail.com and the SSL option should be enabled.
*Older sites are using this plugin. Newer sites will be using the Postman SMTP plugin http://wordpress.org/plugins/postman-smtp