There is no firewall between the lan and the mail server. They are both behind a firewall which separates our lan from the Internet. I'm pretty sure the problem is with the courier smtp server. I can access an adjacent Unix server with sendmail and I get the appropriate response. Please see my latest post. It now appears that the Courier SMTP server is just responding very very slowly, so slowly that I didn't think it was responding at all and any attempted communication fails due to a time out. The Courier pop3 and imap servers respond promptly.