The main cause for this issue, is that your internet connection has been blacklisted, which causes the mailserver to reject the connection when sending, to resolve:
- Visit http://www.ipchicken.com and take a note of your "Current IP Address"
- Visit http://mxtoolbox.com/blacklists.aspx and enter the IP address, click "Blacklist Check"
- If your IP address appears in either: Spamhaus ZEN, BARRACUDA or CBL then this is the reason you cannot send email.
- Clicking the details button next to any listed Blacklist with take you to the Blacklists website, where you can request removal. Please note that the website will detail why the IP was added to the blacklist, and this issue should be resolved before requesting removal. The biggest reason for being added to blacklists is a virus on one of the computers in your network.