If you are having trouble sending mail but not receiving it, there is a possibility that your ISP is blocking port 25, which is the port used for outgoing mail. Providers like AOL block this to prevent outside users from relaying bulk email off their servers. You should contact your provider to see if port 25 is blocked. Otherwise you have incorrectly entered details in your email client for the outgoing server(SMTP) for example make sure you are authenticating the outgoing mail server as described in other support database articles, or software on your machine like anti-virus, firewall, Norton, Symantec, etc is conflicting. If port 25 is blocked, you could try using port 587 instead, as this is an alternative port for the SMTP connection.
If this is not the issue, then your IP may have been temporarily blacklisted, or possibly the IP of our server, as some spam blacklists can become a little overzealous from time to time. If you contact our support team they can usually help to get this resolved for you, or advise the best ways to prevent it from affecting you.