Phishing emails

[#1]

On some occasions when I try to report a phishing email by forwarding to the relevant department of the organisation concerned the Plusnet server rejects it with the following message:-
"An error occurred while sending mail. The mail server responded:  5lbv1p0062HP6L701lbwEN message rejected due to spam or virus. If you believe this is in error please login to your portal or contact your ISP support team.. Please check the message and try again"
To get over this I usually take screenshots of the email and source and send them as attachments.
My questions are : 1) If the server can detect a problem in the message why doesn't it prevent it getting into my Inbox in the first place? 2) Is there any way to override this detection so I can forward the message as is?
Incidentally my email client is Thunderbird.

[#2]

Quote from: Buggwrit

1) If the server can detect a problem in the message why doesn't it prevent it getting into my Inbox in the first place?

The email may not have been considered a threat at the point of delivery. Both our inbound and outbound filtering platform use real-time threat detection. It's been a while since I looked at it, but the inbound/outbound logic probably differs slightly too.

Quote from: Buggwrit

2) Is there any way to override this detection so I can forward the message as is?

Not easily, no. Having said that, if Cloudmark sees the message as spam then chances are, so will the authority you're forwarding the email too.

[#3]

Thanks for the swift response Bob. I guess I'll just have to keep sending screenshots. This is only necessary on a small proportion of the spam and phishing emails I get and there is no obvious difference, to my eyes, between those that are affected and those that are not. Nor does my anti-virus (MSE) report anything.
Actually the Cloudmark page you linked to specifically states that the same filtering scheme is applied to inbound and outbound mail but clearly there must be a difference in the way Plusnet's mailserver handles it.