error on outlook express
 

when I send messages in outlook express like letters and small attachments
it work fine ,
but when sending attachments like videos I get the message

(Your server has unexpectedly terminated the connection. Possible causes for
this include server problems, network problems, or a long period of
inactivity. Subject 'toot tone', Account: +++++++++++++++, Server:
++++++++++++, Protocol: SMTP, Port: 25, Secure(SSL): No, Error Number:
0x800CCC0F)