Presumably there's no reply after the Server reply - bit then? That means the server's not saying why it's not accepting the mail.
Can you enable server logging on the account and then try again. That should give us the full transcript of the dialogue between you and the server. To enable logging, go to Account > Properties >Transport > Protocol Logging and tick the box alongside the Sending Protocol Enable logging and make sure that SMTP.log is in the filename box. This will create you a text file in the account folder of that mail account - use Windows Explorer to find this and then copy and paste the details into a reply or send to me via PM here on the board once you've tried to send again.