SmtpClient fail to send email using Amazon SES endpoint


I'm using .Net SmtpClient to send emails with Amazon SES.

The emails can't be sent from some PCs and I get a timeout.

I read the Amazon doc ->

I'm investigating if this could be a TLS issue, because the command they ask to run doesn't complete (which is probably causing the SmtpClient timeout issue as well)

openssl s_client -crlf -starttls smtp -connect

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I've run a Wireshark trace on a Working PC

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And then on a non-working PC

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I'm not sure what it means... Any thoughts?


  • Side note, the emails works on most devices in my inventory but does not work on 2 specific computers.

1 Answer


What about doing a packet capture using something like "tcpdump"?
Regarding the "tcpdump" command, this answer may be helpful.

I don't think it's very likely, but could it be blocked by the OS's firewall?

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answered 5 months ago
  • Using Wireshark, I see there is a difference in the network trace between a working/non-working environment. See my Edits with screen capture in my original post.

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