Did you check your Filter Rules to verify you don't have a rule to file email from firstname.lastname@example.org into your Spam folder?
Ways for Spam to get into Webmail Spam folder
Set Spam Settings to: "Automatically deliver to Spam folder" (not default).
Create Filter Rule to file email from a specified address into Spam folder.
Open an email and manually select "Mark as Spam".
Manually move email to the Spam folder.
There appears to be a discrepancy in what you're saying or omitting. You've indicted Spam Settings are set to "No Spam filtering". For email to be in the Webmail Spam folder, some other user action had to have been taken.
How are Forum notification emails getting into your Webmail Spam folder?