Roundcube confusion in a multiserver setup

[webguyz]

I installed Roundcube on a web server (web2.domain.tld) in a multiserver ispConfig network that has all email on a separate server (email.domain.tld). I have setup a vhost alias so all webmail users on that server web2.domain.tld can access their webmail by using http://their_domain.com/webmail

This works well and they can receive email with no problem. Outgoing email is whats confusing me. I have this variable set: $rcmail_config['smtp_server'] = 'localhost'; (tried '%h' but was getting send errors) Not getting any errors when a Roundcube user sends email and it says that the message was successfully sent, but it never gets to the recipient. I don't see anything in the mail log about the email I was trying to send getting an error, don't even see it mentioned anywhere. Where the heck is Roundcube sending these emails to? They show as being sent in Roundcube. I even looked on the web2.domain.tld server where RoundCube app is installed but don't see it anywhere in any error logs.

Anybody out there can clear this up for me?

Thanks!