can receive but not send emails.

I have been unable to send emails for the past few days.. I have never had a
problem before and I have checked all ofmy settings- the seem to be ok.. I am
recieving emails.. this is the message I am getting.. help!

The connection to the server has failed. Subject 'twsting', Account:
'', Server: '', Protocol: SMTP, Port:
25, Secure(SSL): No, Socket Error: 10060, Error Number: 0x800CCC0E