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Is there a filter I can apply to capture the SMTP tracing from the SMTP server?
Your diagram is unclear. As Jaap mentioned, it implies that the printer has two interfaces and switches between the hub and the Comcast device. Are you sure that the PC and printer are really connected by a hub? True hubs are rare these days, and many devices labeled as hubs are actually switches. Also, some dual-speed hubs only act as hubs between the ports operating at the same speed (all the 10 mbps ports; all the 100 mbps ports), but switch between ports operating at different speeds. See here on the Wireshark FAQ page for a discussion.
Your filter is correct, assuming you're using the correct server address, so if you're not seeing any SMTP packets, it's because they aren't there. Either the printer isn't sending packets to that address, or you're capturing at the wrong place in the network, or the hub is really a switch so packets aren't being sent out the port that the PC is connected to. If that's the case, you could replace the "hub" with a true managed switch that does port forwarding and then forward the port where the printer is connected to the port that the PC is connected to.
