Error 552 Spam message rejected

[#1]

It's clear that this problem isn't new or unique, but rather than add to an old thread I'm starting a new post just to check whether there has been any change in the explanation and possible solutions.

I have a Plusnet email address.  It works perfectly well.  I receive emails from (almost) all my contacts with absolutely no problems.

One of my contacts and colleagues is however getting 'undeliverable' service messages when he send, or copies, emails to my address. 

I don't a have copy of the full message including headers or server details.  But the actual message is

"From: "mailer-daemon@aol.com" <mailer-daemon@aol.com>
To: [my colleague]
Sent: Friday, 15 December 2023 at 15:58:00 GMT
Subject: Failure Notice

"Sorry, we were unable to deliver your message to the following address.

<[my plusnet email address]>:

552: Spam Message Rejected"

I have edited both of the personal email addresses out as this is a public forum.  But a key point is that my colleague uses AOL (one of the very few I know who does so).

My colleague's address is in my Thunderbird address book as a contact and I have checked that it is thus 'whitelisted'.  So I am assuming that his address, or his inward messages to me, are not being blocked by the settings on my email client.

I see however from Bob Pullen's advice in this 2018 forum thread  that the error 552 message arises

"..... because our [Plusnet's] anti-spam filters:-

Don't like the address the message is being sent from.
Don't like the server the email is being sent from.
Don't like something in the content of the email/signature block."

Hmm, so Plusnet doesn't like that AOL address, or some other aspect of the message.  I can't get my colleague to change to a new address, and he can't influence (doesn't begin to understand) what servers his email uses. 

I will check out the third aspect by getting him to send me a plain or blank message with no attachments, links, signature blocks or other extraneous stuff.  But I hope there is some way of reestablishing normal email contact without requiring him (or me) to set up new and different email addresses.

[#2]

Are these chain emails by any chance? The reason I ask is a friend had this issue with another isp several years ago. I determined it was cause by the number of email addresses in the forwarded email activating spam filters at server level. Understandably. 

Get him to send you an email with test in the subject and test as the message. If that gets through ok then that will be the cause.

[#3]

@br1anst0rm There are many reasons why Plusnet may reject emails. At one time during the past 15 months they were rejecting ones from North Wales Police to me, and though PN were supplied with headers couldn't find out why they were being rejected. ( TheNWPolice IT guy asked me who was the email hoster/ISP and when he heard PN, said they have issues with them.) LinkedIn emails that are automatically forwarded from my Gmail account normally get rejected.  It doesn't bother me as I LinkedIn isn't one of my priorities. 

Another reason could be if PN assumes DKIM is in operation and if any part of that is incorrect then it could be rejected.  I don't know if PN does enforce DKIM.  Sometimes it can be if a senders email address has been reported as a spammer, etc and that information is normally added to the abuse lists that most if not all ISPs, etc, use  (I do report spam etc to spamcop.)  That's not the only way email hosts detect spam, etc. You could go to the mxtoolbox.com site and check the DKIM of your colleagues AOL address as well as see if it's on any of the blacklists. That would remove those from the scenario of issues, or possibly point you in the right direction to investigate.  Working your way through the header can sometimes give an indication, though if you don't know what you are looking for can give you a headache.  There are other posts on the email section of the PN community that may help, as well as online.

[#4]

Thanks to both @iannewson and @Total_Chaos for those replies.

I haven't quite pinned down the reason for the problem that emerged, but I have narrowed things down a bit.

The original email which was rejected and labelled as spam by the Plusnet servers was not a chain email.

Looks as if my colleague/contact's AOL email has not been "blacklisted" or identified as a source of spam.

A simple test message from him with no links or attachments came through to my Inbox perfectly normally.

But the original message had - as an attachment - a complete web-page (an .html file). That web-page was from a perfectly legitimate website (a blog commenting on government policy).  But the web-page had links and addresses within it (and maybe other "active" elements).  So I have to assume that the filters on the Plusnet servers didn't like it, and so labelled it, incorrectly as spam.

Now that the mystery has been more-or less solved, I'm more relaxed.  I had been worried that Plusnet might have been blocking or filtering out other normal and harmless emails addressed to me which the senders had not realised or thought to mention to me.  Thankfully that seems not to be the case.

It seems that Plusnet filters are more rigorous than some others.   And I suppose it's better for Plusnet filtering to challenge the occasional genuine message rather than to allow lots of spam messages through....   But in some ways I would rather Plusnet adopted the approach of other ISPs, and allowed suspicious emails through but with a large "SPAM" warning flag in the subject line, rather than rejecting them without informing me.