how do i fix this? can't send emails!

what does this means:
An unknown error has occurred. Subject 'Christensen & Kockrow: Foundations
and Adult Health Nursing, 5th Edition', Account: '', Server:
'', Protocol: SMTP, Server Response: '421 Cannot connect to SMTP
server (, connect error 10060', Port: 25,
Secure(SSL): No, Server Error: 421, Error Number: 0x800CCC67