552 Spam Message Rejected 221 avasin[0-9][0-9].plus.net Plusnet Cloudmark Gateway

[#1]

When our company's system sends to a plusnet address we often see the response in the Subject of this Post.

Cloudmark ( https://csi.cloudmark.com/en/reset ) say for our mail server address:

Submitted IP Address is Not Listed in Cloudmark Sender Intelligence

The SMTP rejection that you have received is not the result of current data provided by Cloudmark Sender Intelligence.

Can you help us get these legitmate mails to our (and plusnet's) customers?

[#2]

Hello? Anyone there? For some similar posts admins offered to assist via private messaging but I don't know how to send such a private message to an admin...

These mails are definitely legitimate but we continue to see (some details replaced by ...)

QUIT250 <...> sender ok
250 <jeremy@...plus.com> recipient ok
354 OK
552 Spam Message Rejected

221 avasin10.plus.net Plusnet Cloudmark Gateway closing connection

And Cloudmark continue to deny our IP is listed.

[#3]

Following some back and forth with Cloudmark Customer support they have said:

Hello Jeremy,

The sample you are providing tests as legit. As does the IP you previously show.

You need to contact Plusnet to identify why they are rejecting your message, they will have local logs to exactly identify the reason. If its agreed this is a false positive they can use the tools we provide to remediate or escalate to the Proofpoint Support team

In this case there is insufficient evidence for us to give a reason for problem - this may not even be a Cloudmark Authority issue given or tests are not giving a spam verdict.

Please can you direct me to the correct route to get this looked at?

[#4]

Following further testing and analysis, it appears that Plusnet email servers reject a dash character which under certain circumstances appears as  "~@~S" (eg, WSL SSH + vi ) but in most circumstances (eg, Windows Notepad, iTerm2 SSH + vi) appears just as a dash... ...I am asking developers if their email templates might be modified but it is notable that many other mail servers on the internet we have worked with are not rejecting this, and I suspect Plusnet email servers might be outdated in their handling of newer character codes.