Support queries shared: ‘NOT SCANNED’ messages in quarantine folder

As you can imagine, this one's got the potential to cause a fair amount of head-scratching - I see it quite a lot too. Symptoms are that messages are being incorrectly quarantined by MDaemon's security module SecurityPlus even though the message is from a trusted sender and the attachment is clean of any viruses. Typically when we look into these examples there is one common factor.