unable to send emails due to not connecting to outgoing server

I have been unable to send emails for several weeks and keep getting the following error message: sending of message failed The message could not be sent because connecting to the outgoing server (SMTP)smtp.1und1.de failed. The server may be unavailable or is refusing SMTP connections. Please verify that your outgoing server (SMTP) settings are correct and try again. My settings have not been changed so I am wondering what has changed and how i can correct it. My computer knowledge and ability is very limited so please bear theis in mind when offering advice and solutions. Thank you.