[horde] Connection refused "localhost:587" when sending mail

[Advrk Aplmrkt]

I am trying to setup a groupware server running on Scientific Linux 6
(which is literally RHEL 6 minus the branding) for personal use.
So far I've setup a test virtual machine, running fully updated SL 6,
running Kolab groupware 2.3.2, which has Horde 3.3 as a web interface.
I have been able to setup a user and login to the built in Horde 3.3
web interface (all from both within and outside of my local network).
However, when I try to send a test email from the web interface, I get
the following error:

"There was an error sending your message: Failed to connect to
localhost:587 [SMTP: Failed to connect socket: Connection refused
(code: -1, response: )"

The problem is, I completely *disabled* the firewall from the Firewall
GUI program in the Administration menu, and I don't know why it seem
to have permission problems, or ports being blocked.

What should I check and change to fix this problem? And what does this
error mean anyway? Thanks for your help.