[horde] Mails sent from Horde identified as spam on the sending server

[Volker Then]

Hi,

some of my messages sent with horde (webinterface or ActiveSync  
client) get already filtered as spam by the local spamassassin instance:

Content analysis details:   (5.5 points, 5.0 required)

  pts rule name              description
---- ---------------------- --------------------------------------------------
  3.6 RCVD_IN_PBL            RBL: Received via a relay in Spamhaus PBL
                             [80.131.175.177 listed in zen.spamhaus.org]
  0.2 CK_HELO_GENERIC        Relay used name indicative of a Dynamic Pool or
                             Generic rPTR
  1.6 RCVD_IN_BRBL_LASTEXT   RBL: No description available.
                             [80.131.175.177 listed in bb.barracudacentral.org]

It is due to the Received-from header in the horde emails:

Received: from p5083AFB1.dip0.t-ipconnect.de (p5083AFB1.dip0.t-ipconnect.de
  [80.131.175.177]) by ********* (Horde Framework) with HTTPS; Thu, 01

It is weird that the client is identified as a relay, it shouldn't be  
like this. Is that more an issue of a wrongly configured spamassassin  
or can that be fixed by horde itself, by omitting the client specific  
received-header? If yes, how would I do that?

I have that issue now with almost any dynamic client IP.

Regards and thanks in advance,

Volker