The SMTP server returned an error when connecting or sending mail.
There are many possible causes, depending on the server and its configuration. The error message will contain the error as returned from the server itself, which should help point toward a fix. If the message contains no more details, such as simply saying "Failure sending mail" then there may be additional information which can be viewed by clicking "More Info" and then expanding the Details section.
If the error information doesn't point a way toward the answer then submit the error to support using the More Info window.