only admin can receive emails, everyone can send - Citadel

When I send an email to a user created in the Citadel web interface. I get the following error

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Delivery to the following recipient failed permanently:

tson@somedomain.ca

Technical details of permanent failure:

Google tried to deliver your message, but it was rejected by the server for the recipient domain somedomain.ca by somedomain.ca. [XX.XXX.105.XXX].

The error that the other server returned was:

550 Invalid recipient: tson@

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I can send emails from the Citadel web interface as user tson and the email goes to Gmail correctly and it says it is from tson@somedomain.ca. But when I click reply or when I try to compose an email to tson@somedomain.ca and send it, I get the above error message from Google. The admin user can send and receive email.

I installed Citadel on my linode using the instructions for Debian 6.

When creating the user I check the box to allow webmail. I also tried disabling Spamassassin without any luck. I followed the suggestions in other posts on this site but without any luck.

How do I add recognizable users so that Citadel will allow them to receive email?

Any help would be much appreciated.

Bill Dika