Email worked when I set up my account but no longer works

When I try to send an email here is the message I get. Sending of the message failed. The message could not be sent because connecting to Outgoing server (SMTP) mail.sasktel.net failed. The server may be unavailable or is refusing SMTP connections. Please verify that your Outgoing server (SMTP) settings are correct and try again. I checked my outgoing SMTP settings and they are all what they are supposed to be. Note** when I first installed Mozilla everything worked fine. Now I can not receive or send out emails.