Invalid user when sending emails

I have a Ubuntu server with Postfix and Courier (both appear to be running fine). We used to have a control panel provided by our host but after upgrading the server it's no longer available. But now all of our emails are bouncing.

We host several domains and several email addresses per domain. When I send email to e.g. [email protected] it returns a failure notice:

The mail system
<[email protected]>: user unknown. Command output: Invalid user specified.

[email protected] is not an actual user but between Postfix and Courier I think I traced where it comes from. The virtual domains file /etc/postfix/virtual shows lines like this for each email address (first column should be the email address, second column the user):

[email protected]    [email protected]

Now in the folder /etc/courier/userdb/ there are files for each domain on the server. Inside these appear to be "virtual users" that correspond to the emails addresses set up. In the example.com folder is this:

[email protected]       uid=1000|gid=1001|home=/home/example/example.com/system/mail/users/scott|mail=/home/example/example.com/system/mail/users/scott/Maildir|gecos=scott|systempw=$1$M0USmU7K$2f/KbNOLOdVqp.Ra4gKXR/

(The example in /home/example/ is just a common user that "owns" several sites.)

My knowledge of Courier is almost non-existent but at a glance it seems like everything is set up. Is there a missing link somewhere between Postfix and Courier? What methods are there to diagnose why the mail isn't working?

Let me know if I can provide any more information.