Outlook Express. An unknown error has occurred

[DaireMacAdaim]

Hello everyone,
I am using Outlook Express and when im trying to send emails I get this error:
"An unknown error has occurred. Account: 'Scoil Eimear', Server: 'scanner.scoilnet.ie', Protocol: SMTP, Server Response: '421 4.7.0 Not allowed.', Port: 25, Secure(SSL): No, Server Error: 421, Error Number: 0x800CCC67"
Please help me to fix this error.
Regards

 

[DelJo63]

That's a temporary, transient SERVER condition,
421 The service is unavailable due to a connection problem: it may refer to an exceeded limit of simultaneous connections, or a more general temporary problem. The server (yours or the recipient's) is not available at the moment, so the dispatch will be tried again later.