[Citadel Support] "invalid recipient"

[kjoiwerjcoa]

I have installed Citadel on a Ubuntu 8.04 server, but I cannot send email to
anyone.  An error appears in the upper right-hand corner which states
"Invalid recipient: [any email address i try sending to]."  I've tried
localhost, 127.0.0.1, my actual domains, and other email accounts such as
yahoo.  The message doesn't even go to the queue.  I've tried uninstalling an
reinstalling three times without any errors.  I've tried googling and
searching this site without any luck.