This is the error message I receive when sending mail by Windows Mail

Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Subject 'test 1', Account: 'inbound.att.net', Server: 'Outbound.att.net', Protocol: SMTP, Port: 465, Secure(SSL): Yes, Error Number: 0x800CCC0F

Can someone assist in this?