Some clients were experiencing issues with the email system. A new update was just pushed out ( Saturday, June 14 @ 13.10 hours PDT ).
Please go to Tools -> Setup -> Email
Make sure to check ONLY the box for your type of email interface ( SMTP, MAPI, GMAIL ).
Select the Apply button
Now test your system. If you are getting an error message, and you are using SMTP, make sure the settings in that section of the email setup are correct. Your provider may have made changes.
Again, after making some adjustments, we have tested all 3 connectivity methods and found them working successfully.