valid messages being rejected as spam

[#1]

Today, Fri 20 Aug 2021, I've been trying to send both simple (short) messages and messages with PDF attachments from my work account to my home (Plusnet) email address.  They are all being rejected as spam. My work IT people tell me it's Plusnet that are doing the rejecting.  I've never had this problem before.

What do I have to do in order to once again receive messages from my work account?

stuck

EDITED TO ADD:

The PDF files involved are all small, one is only about 150KB, the other three are each about 1.5MB

[#2]

There's usually more information in the bounce message as to why the message is being detected as spam.

Sight of the relevant part of the bounce may help ( redact any personal information )

[#3]

OK but I think deleted all the bounces.  I will check the next time I boot my work laptop though, but that won't be until Monday morning.

stuck

[#4]

The bounce message says:

host mx.avasin.plus.net [212.159.8.200]

SMTP error from remote mailserver after end of data:

552 Spam Message Rejected

[#5]

There's usually more information with the 552 error ...

[#6]

There isn't.

If you Google the IP address, you get lots of hits about Plusnet rejecting genuine messages e.g.:

in 2016 https://community.plus.net/t5/Email/Legitmate-emails-to-plusnet-blocked-due-to-spam-causing-issues/t...

in 2020 https://community.plus.net/t5/Email/Email-rejected-as-spam/td-p/1716119

From those threads, it seems that Plusnet can fix this problem so please can I ask that you use you "direct line of communication into the business in order to raise issues".

Thanks,

stuck

[#7]

I've escalated the thread to attract better visibility to Plusnet staff.

[#8]

Thanks.

[#9]

There are usually two possible reasons for a message to be rejected as spam by the Cloudmark system, one being the message content and two the reputation of the sending domain. You seem to have eliminated the first be sending different types of messages. Given that, in the mean time it might be worth getting your IT people to request their IP reputation is checked by Cloudmark by completing the reputation remediation form here https://csi.cloudmark.com/en/reset/

[#10]

Hi @stuck, thanks for getting in touch and I'm sorry to hear you've had issues with the spam filter when trying to send emails through to your Plusnet email recently.
There's a number of reasons the spam rejection could be happening.
From the sounds of it you've already ruled out the size limit of the content from being the cause. I'd advise if you have any URL's in the sent message to perhaps try removing/breaking it up and seeing if the email goes through also if you haven't already.
As @MisterW advised, the other cause of the spam filter being flagged is due to the reputation of the sending IP address. This could be caused by the IP having previously been used and flagged for spam or if enough mail that's being sent out from the IP address is flagged as spam this can trigger the change in the IP reputation.
I'd advise then initially to check your works IP address on the spam filter we use here:
https://check.spamhaus.org/
On top of this as advised, should you look to have an incorrect reputation associated with your works IP address then this can be put forwards for review via the link provided in the last comment.

Hopefully this helps in narrowing down the cause of the spam bounce back message you're seeing and please do let us know how it goes.

[#11]

I have already begun to experiment will experiment with sending messages of increasing 'complexity' to see if I can pin down what caused these bounces.  I will report back in due course but it may take a while.

My work domain is a large organisation that does business all over the world.  When I enter the domain into the spamhaus link, it comes back "...has no issues".  I would have been very surprised if it had reported anything other than that.

stuck

[#12]

If it helps @stuck feel free to PM me and I'll send you my email address that you can send to from work and I can extract the spam filtering results from my server that may shine a light on what aspects of your message might be deemed suspicious.