SMTP Error: Failed to add recipient

[N P List]

Hi All,

I recently finished the Roundcube setup (version 1.4 - I think).  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