TFL are rejecting authenticated mail relayed through our servers due to one of the SMTP Received: headers containing an IP address listed at Spamhaus.

In this case, it's the mistaken & inadvertent use of the Spamhaus PBL where the error lies. A majority of e-mail sent from a home broadband connection will contain an IP listed in the PBL. TFL (or is it Capita?) are wrongly using the Spamhaus ZEN blocklist which is an aggregate of all the lists maintained by Spamhaus. This includes the PBL.

Here's the rejection message:

  REDACTED@tflcc.co.uk
    host smtp.tflcc.co.uk [80.82.130.162]
    SMTP error from remote mail server after end of data:
    550 5.7.1 92.40.249.10 listed at zen.spamhaus.org

The IP address TFL take an exception to is [92.40.249.10] which is indeed listed in the PBL. The crucial thing is that this IP has not connected to TFL's servers at all. Instead it has been plucked from the trace headers included in the message.