This error can mean several things. The most common though is what is known as a "ghost connection". This connection is just one that has not terminated from a previous transaction with the server. This could be coming from you, or from a message being delivered that hasn't finished. There are several steps you can take to resolve this.
1. Restart your mail client and try again.
2. Exit your mail client and wait 10 minutes then try again.
3. Restart your computer and try again.
4. Call our support at (972) 294-3867 or email support@cfdynamics.com to request that we manually unlock your box.
This should not affect your outgoing email, so any messages you are trying to send should still be able to go through. Please note that the response time from the server when your email client is getting these errors can be considerably longer.