[imp] Horde (IMP) email problems

[Louis-Philippe Allard]

Hello IMP maillist,

I have installed Horde webmail onto a brand new CentOS 6.5 server, and
experience a few major issues with sending emails.  IMP is configured as
the login backend and the imp conf.php file contains more or less the same
content as my previous working install of horde so I expect the connection
parameters to GMail's SMTP servers NOT to be the culprit of my issues.

A quick description of my issues:

When I create a new email, I press "Send" and see a green tag at the bottom
of my screen saying "Message **** sent successfully"

So far I have tried sending emails to two different adresses, one on Gmail,
and the other on hotmail.  None have made it through.  Sending an email
from my horde to a hotmail address, I get a mail from "Delivery subsystem"
with more or less this content:

***********************************

"The original message was received at Thu, 8 May 2014 08:14:25 -0400
from localhost [127.0.0.1]

   ----- The following addresses had permanent fatal errors -----
xxxxxxxxxxxxxx at hotmail.com
    (reason: 550 OU-002 (COL0-MC6-F27) Unfortunately, messages from
XX.XX.XX.XX weren't sent. Please contact y...ock list. You can also refer
your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.)

   ----- Transcript of session follows -----
... while talking to mx4.hotmail.com.:
>>> MAIL From:<lp.allard.1 at gmail.com> SIZE=1839
<<< 550 OU-002 (COL0-MC6-F27) Unfortunately, messages from XX.XX.XX.XX
weren't sent. Please contact your Internet service provider since part of
their network is on our block list. You can also refer your provider to
http://mail.live.com/mail/troubleshooting.aspx#errors.
554 5.0.0 Service unavailable

Final-Recipient: RFC822; xxxxxxxxxxxxxx at hotmail.com
Action: failed
Status: 5.0.0
Diagnostic-Code: SMTP; 550 OU-002 (COL0-MC6-F27) Unfortunately, messages
from XX.XX.XX.XX weren't sent. Please contact your Internet service
provider since part of their network is on our block list. You can also
refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors
.
Last-Attempt-Date: Thu, 8 May 2014 08:14:25 -0400
***********************************

Sending emails to a GMail address, the email never reaches the
destination.  Not even for myself (sending an email to myself via Horde).

I have contacted my ISP since the reply from Hotmail pretends that part of
my network is on a blocklist...  My ISP are not aware of the cause and said
that they were not at fault.

I hope someone can help making this setup work...

Thanks and best regards