SMTP Error: Failed to add recipient

[N P List]

Hi All,
I recently finished the Roundcube setup.  I was getting a couple of 
errors, when I try to reply or send mail.  The first error had something 
to do with permissions of logs and temp directory.  I fixed it, thanks 
to some post in the list.  The second error is:
SMTP Error in /srv/www/htdocs/webmail/program/steps/mail/sendmail.inc 
(185): SMTP error: Failed to add recipient

Any idea as what this might be?

Thanks,
Nitin