It is Exim's job to deliver email to mailboxes. Courier's job is to server the messages to email clients like Outlook and Horde. If email is not reaching the mailboxes, you should really be debugging Exim, not Courier.
This is what I would try:
* For a nice independent opinion on the status of an email address, use www.dnsreport.com. Enter the email address there, and see what the response is. If you do not get a 250 (good) response, then there is a problem with the way the domain and/or email address is set up.
* If you get a 250 (good) response, then something on your server is preventing delivery of the email. Likely problem points include your Mail Filters and/or SpamAssassin. You can search your Exim log file for clues, e.g.
Code:
exigrep clientname@clientdomain.com /var/log/exim_mainlog