> When users are forwarding spam to firstname.lastname@example.org to add them to the spamfilter lists as directed to on the quarantine reports we are getting more of these undeliverable errors?
> Subject: Mail Delivery Failure
> Delivery Failure Report.
> The following message was incorrectly addressed.
> Recipient: "spam" is unrecognised.
That error happens when there is incoming mail for an unrecognised user. So, someone must be sending a message to 'spam@' from outside your LAN. and without using SMTP authentication with VPOP3. VPOP3 doesn't recognise 'spam@' as being a special address for incoming messages, otherwise it would be abused by spammers trying to pollute your spam filter training database.
To send messages to spam@ to train the spam filter you need to be sending via VPOP3's SMTP service either as an authenticated user, or from an IP address where authentication isn't required.