Most likely your being attacked.
If your using 16.x or the most recent Build, you can look at Connections in the Manage Section. See if you have many requests coming from a specific IP or if their distributed. For the health of your Mail Server you should attempt to block as much as you can on a Firewall or Networking device. If you can't do that, use SmarterMail's blocking. For future attacks you can configure IDS rules to catch and do these on the fly as well.
Clearly I'm stabbing in the dark based on the information but wanted to give you some ideas. If you Maintenance and Support, our support team can help you as well.