Thanks for the reply. I can disable the SMTP authentication and Roundcube will send outgoing emails. But those emails will not be signed by dkimproxy which is the real intent I am striving to obtain with this setup.
Do you think it might be a permissions issue?
Squirrelmail is installed in /usr/share/squirrelmail and all files appear to be root:root. Then, /var/www/webmail is a symlink to this location.
When I installed Roundcube, I created a new folder under /var/www/client#/web/webmail and created a subdomain pointing to this location in ISPConfig. I uploaded the Roundcube files there and did the configuration.
The ownership of those files though are web#:client#. Could that difference perhaps lead to the problem I am experiencing?