Several possible errors will not allow your e-mail messages to be sent. If your ISP has no modems available, you will get a 'busy signal,' causing an "Error sending e-mail, line is busy." If you are disconnected from your MLS because you are 'timed out' (many systems are strict about inactivity and will disconnect you) before you click the Send button on an e-mail, the mail will not be automatically sent.
You will have to reconnect to your MLS, and log off again to send the e-mail messages.