You can receive e-mails that are spam but have not been labeled as such. In this case we speak of a "false negative".
Spam that was not detected, can be reported. You are thus helping to refine the detection system.
Send Spam messages attached to a new post
Send the Spam message attached to a new post to firstname.lastname@example.org. Enter "spam" as subject for the message.
in webmail: select the spam message and click 'Report as Spam'.
Note: The reason why the e-mails have to be sent as attachments and can not simply be forwarded, is that when a message is sent as an attachment, the full headers are included. The full headers include routing data which we need to deal with the message.
If forwarded, the message could also be held on its way as being spam.