Alright, I can send and receive E-mails as root, however, when I setup an E-mail user via ISPConfig, and send an E-mail to the recipient/user I get the following error:
This is the mail system at host server1.resacahost.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<firstname.lastname@example.org>: user unknown