Plusnet silently drops some emails

[#1]

Plusnet’s spam engine (Cloudmark + PN MXCore)

This is where the nonsense happens.

Cloudmark score:
`
X-CM-Score: 0.00
`
Cloudmark says: not spam.

SpamAssassin composite score:
`
X-Spamd-Result: default: False [-4.15 / 4.00]
`
A score of -4.15 is extremely hammy.
Again: not spam.

And yet Plusnet adds:
`
X-pn-pstn-db:" Spam 99
`

This is a Plusnet‑specific internal flag, not based on industry-standard scoring.
It is known to be unreliable and often misfires

[#2]

In the current ultra leaky spam filter the "spam flag" in the email header / message source is : 

X-Spam: Yes