If you can receive mail but aren't able to send this means one thing.....

There are number of things causing this. Unless you've received an announcement from us that mail isn't working properly at the time you're having difficulty.

Chances are it is some internet provider is blocking you from using anyone's incoming mail servers but their own. Many major ISP's are blocking incoming emails.

The reason they do this is to prevent you from using their internet connection for spamming purposes.

You will still be able to send emails, but only by using your ISP'S outgoing mail server.
Every ISP's outgoing mail address is different, but for the most part they will be mail.yourispname.com
For example, if you had hosting with AOL.com, their outgoing mail server would look like this: mail.aol.com.

If mail.yourisp.com does not work and you are still unable to send emails, you will need to call your internet service provider to find out what their outgoing mail server is.