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[TLS] Re: [lamps] Re: TLS Client Certificates; a survey

John Levine <johnl@ietf.email> Wed, 25 March 2026 16:53 UTC

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Date: Wed, 25 Mar 2026 12:53:43 -0400
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From: John Levine <johnl@ietf.email>
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Subject: [TLS] Re: [lamps] Re: TLS Client Certificates; a survey
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It appears that Wei Chuang  <weihaw@google.com> said:
>> I am very surprised to hear that Gmail presents client credentials in
>> the SMTP-OUT direction?  Why is this done?
>
>We observed that for 38% of outbound messages, the server requests client
>the certificates.

I believe it, but I also agree with Viktor that's not a reason you should send them.

My semi-informed guess is that it's a combination of a poorly chosen default in some
MTAs, and hosts that use client certs to log in for submission.

It'd be interesting to hear if you can tell what MTA software is making the client requests.

R's,
John

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