GMX Mailer Daemon Failure - 553 Message Filtered

[#1]

I started to experience emails from my GMX account being returned due to the same failure: 553. Only two emails so far. Interestingly enough it started to happen the day after my fixed contract with PlusNet finished and I went onto a monthly payment plan. Perhaps this is a transfer of information from one account to another that is causing this? Anyway this is what I'm getting.
The Mailer Daemon notes the following: -
SMTP error from remote server for TEXT command, host: cluster8.eu.messagelabs.com (85.158.139.51) reason: 553-Message filtered. Refer to the Troubleshooting page at
553-http://www.symanteccloud.com/troubleshooting for more
553 information. (#5.7.1)
When you access symantec cloud it sends you to Spamhaus Project which provides the ability to look at IP addresses that the account has been sent from. Wasn't aware what these were as they don't actually show on my email account set-up on the computer. When I checked out one of the IP's it noted the following: -
The IP address I gave them is listed on the Policy Block List (PBL)
Outbound Email Policy of Plusnet Plc for this IP range:
It is the policy of Plusnet Plc that unauthenticated email sent from this IP address should be sent out only via the designated outbound mail server allocated to Plusnet Plc customers. To find the hostname of the correct mail server to use, customers should consult the original signup documentation or contact Plusnet Plc Technical Support.
Any ideas whilst I tackle the Accounts team to see if this is an issue from their side?

[#2]

Your IP will and should be on the Spamhaus Policy Block List if it is a dynamically allocated one which changes every time you disconnect and reconnect to Plusnet. As such it is not suitable for sending mail directly to an addressee's email server and it seems the messagelabs scanner has rejected it on that basis.
Mail from dynamic IPs has to be sent via the Plusnet SMTP server relay.plus.net (assuming you are a Plusnet customer) which has the means to verify your identity or an alternative relay with that capability.
Since this doesn't happen every time have you noticed anything specific to the emails that do fail?