Emails to plus.net and plus.com recipients rejected with 552 Spam Message Rejected

[#1]

I am posting as an email sender, rather than as a plusnet customer. I represent a local council sending weekly email newsletters to residents, who have subscribed for the service. All emails to plus.net and plus.com customers are being rejected with the (redacted) message: 

Diagnostic-Code: smtp;552 Spam Message RejectedStatus: 5.0.0 (undefined status)
Action: failed

X-PowerMTA-BounceCategory: spam-related

Is there a way of getting my email sending whitelisted? Thanks in advance.

[#2]

Thanks for your post @idh and welcome to our Community Forums.

I'm sorry to see you're having issues sending emails to our customers.

Can you private message me a full copy of the headers of the email you're trying to send so I can take a closer look?

[#3]

Also seeing this with Postmark-sent mail for f9.co.uk and force9.co.uk email addresses. It's a nightmare.

[#4]

I'm sorry to hear that you're also experiencing this issue @ms7821. If you haven't had a chance yet, please also send me or @Gandalf a Private Message with the email headers and we'll be happy to review the case further.