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Unable to see SMTP traffic after getting new laptop

asked 2021-01-18 22:46:49 +0000

kb gravatar image

I was always able to scan for SMTP traffic when troubleshooting a MFP scanning to email issue on my laptop when using a HUB or Port mirrored switch. I then got a new laptop and now I am unable to see SMTP traffic when trying to scan to email from an MFP. I can still see everything else, ICMP, SMB, TCP and all else. When I try to scan to an email I cant see any DNS, SMTP, TLS or anything. It's as if nothing happened at all but the scan successfully sends to an email so I know it passed through the network. Anyone have any idea why?

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What operating system did the old laptop run, and what operating system does the new laptop run?

What network interfaces (Wi-Fi, Ethernet, mobile phone modem, etc.) did the old laptop have, and what network interfaces does the new laptop have?

Guy Harris gravatar imageGuy Harris ( 2021-01-18 22:51:45 +0000 )edit

Sorry I'm doing three things at once and I keep messing up my reply. Both were Windows 10 but the old laptop was upgraded from Windows 7. Both laptops as far as I can remember (the old laptop is gone now) have the same interfaces. Ethernet and a wireless card.

kb gravatar imagekb ( 2021-01-18 23:04:25 +0000 )edit

So are you capturing on Ethernet or on Wi-Fi? From "when using a HUB or Port mirrored switch" it sounds as if it'd be Ethernet.

Guy Harris gravatar imageGuy Harris ( 2021-01-20 03:30:13 +0000 )edit

Do you mean you see the ICMP and SMB traffic that comes from / goes through the IP address of your MFP device in your capture, but not the SMTP traffic? In this case I would guess that your capture setup is fine, and a kind of new installed antivirus-firewall-proxy-ips-vpn-solution is blocking the SMTP traffic before it reaches the capture driver. Or you've just disabled the SMTP protocol. Open the enabled/disabled protocol dialog (CTRL+SHIFT+E) and search for SMTP.

If you mean that you can capture ICMP and SMB in generally, I would suggest to check your capture setup. Start a capture and send an ICMP request to your MFP device from another PC. Do you see the ICMP request and response in the capture?

JasMan gravatar imageJasMan ( 2021-01-22 22:31:43 +0000 )edit

It was the anti virus! I disabled it and magically all SMTP traffic showed up. Thank you very much JasMan!

kb gravatar imagekb ( 2021-03-24 23:43:38 +0000 )edit

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answered 2021-03-24 23:44:16 +0000

kb gravatar image

It was the anti virus! I disabled it and magically all SMTP traffic showed up. Thank you very much JasMan!

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And for reference, what was the AV program that caused the issue?

grahamb gravatar imagegrahamb ( 2021-03-25 08:29:02 +0000 )edit

Symantec Endpoint Protection.

kb gravatar imagekb ( 2021-03-26 16:49:43 +0000 )edit

You should report this to the npcap folks over at, I don't know if they can do anything about it though.

grahamb gravatar imagegrahamb ( 2021-03-26 17:42:32 +0000 )edit

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Asked: 2021-01-18 22:46:49 +0000

Seen: 87 times

Last updated: Mar 24