Email wrongly bounced as spam

[#1]

PN is bouncing emails as spam when they are not. Gmail accepts them.

They were our own so no third party involved.

A test email without an attachment was fine.

Poor service, PN.

[#2]

Still happening.

PN is bouncing Gmail with attachments. Gmail to Gmail and other services is fine.

PN service gets worse.

[#3]

@Mardler HI, I should let you know that we aren't actually improving our email platform anymore as we no longer offer it to new customers and preventing the existing broadband customers from keeping it open once they leave and as such aren't actively upgrading it to the standards that Gmail and other main stream providers use but never the less if you could send over the header information of the bounce back we can see if it tells us the problem.

Feel free to DM us if you rather the personal info be in the public domain. 

[#4]

@Mardler 

Much more detail might be helpful and very relevant.

What attachments, is this redirected email and is the forwarding service using SRS?

On various mail environments there are known issues where their forwarding service corrupts the headers in a forwarded email which results in the receiving service deeming them to be spam.  For example see Issue - Emails forwarded by Plesk fail DKIM checks with certain attachments | Page 2 | Plesk Forum

In general, if a forwarding service is not using SRS then it is most likely that the email will fail SPF spam checks.

[#5]

Thanks for your comment, TM, & apologies for not replying sooner as have been laid low by a bug.

As I said in my OP, the attachment was mine (I omitted to add that it was a jpeg, one of my own photos) thus was also my own email not a forwarded one.

Over a month ago now and no recurrence but I have no doubt that it will happen again.