postfix - Authenticated outgoing email is marked as spam by PBL on mailserver

Users are sending email, authenticated, through the
submission port on my mailserver (their domain MX record points to mailserver;
postfix).

What's been
setup

• A
  record


• MX record (pointing to same mailserver for all
  domains)


• PTR record resolving to mailserver
  name


• DKIM: pass


• SPF:
  pass


• DMARC:
  pass


• MailScanner with clamd and
  spamassassin


• SASL authentication (mail headers mention
  user is authenticated)



• No open
  relay


• ...

I
see that mails are authenticated in the
headers.

However I see that spamassassin marks
it as spam (it mentions that the IP of the client is on the RBL). When I query spamhaus
I see that the client IP (which is dynamic due to mobile ISP).
Zenhaus says
it's on the PBL, so basically it is marked as spam as a policy based on the client
IP.as

Apart from that there's nothing wrong with
those emails. The other ISPs don't have this problem and the emails are then delivered
properly.

Now on to my questions...
:)

• is mail send through
  submission port supposed to go through Mailscanner (spamassassin + clamd)? I would
  suppose yes as it would already prevent people from sending spam in the first place
  (instead of preventing spam email to be delivered). On the other hand a receiving
  mailserver can't trust what's in the headers so it'll probably check it
  anyway.


• Is there a way to not mark as spam if only
  mentioned on the PBL?


• Will releasing the message make it
  deliverable? Or will it just move the problem? (so the receiving mailserver might check
  and mark as spam due to the PBL) If it moves the problem it doesn't seem a valid
  solution to try to bypass the PBL for authenticated
  users.


• Will a receiving mailserver only check the last
  header (so the header added by my mail server)? In this case disabling spam check might
  actually resolve the issue and not move it on to the next
  machine).


• another thing that comes to mind is
  removing/modifying the first header so the IP is no longer mentioned. However this seems
  like a bad practice.


• What's the proper/appropriate way to
  handle
  this?

For
clarity: the mails are received on the smtp server that the users have configured on
their laptop/mobile and put in the postfix queue. So no direct rejection to the clients.
Only after mailscanner jumps in and checks the email before sending (in this case
marking it as spam and not sending it).

Thanks a
lot in advance!