This counts CIFS or NetBIOS Messenger service pop-up messages the gateway blocked from passing through. This specifically tracks a once-common spam or malware vector, unsolicited network pop-ups sent to unpatched Windows machines, so a nonzero count indicates something on the network is still trying to send these, though on a modern estate it's usually low-value background noise rather than an active threat. For example, this counter picking up a handful of blocked messenger pop-ups from an old legacy server still configured to broadcast maintenance notices this way.