Combined ServerAuth+CientAuth for SMTP connections
Roman Fischer
Dear Group,
I didn't participate in yesterday's call but would like to contribute a bit to the question regarding the impact of ServerAuth-only TLS certificates:
One of our customers highlighted the fact that e.g. Microsoft Exchange seems to give SMTP connections that present a client cert that is public trusted (aka TLS certificate with ServerAuth+ClientAuth) a higher rating than connections without such a certificate. They are now of course anxious that their email delivery may be impacted if the ClienAuth EKU goes away.
We tried to contact Microsoft about this but unfortunately have not gotten a response yet. Maybe we could try to get a representative from Microsoft to push this further?
Kind regards
Roman
Roman Fischer
Information Security Manager
+41 76 310 12 66
SwissSign AG
Sägereistrasse 25
Postfach
CH-8152 Glattbrugg
swisssign.com
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Sent: Thursday, February 12, 2026 6:34:43 AM
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Subject: [Smcwg-public] Combined ServerAuth+CientAuth for SMTP connections
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大野 文彰
Dear Roman,
I would like to agree with your concern.
I support the proposal to actively engage Microsoft and obtain a clear statement
on whether, and how, the ClientAuth EKU is used in SMTP connection evaluation.
Best regards,
ONO Fumiaki / 大野 文彰
(Japanese name order: family name first, in uppercase)
SECOM Trust Systems CO., LTD.
From: 'Roman Fischer' via S/MIME Certificate WG - Public (CA/B Forum) <smcwg-...@groups.cabforum.org>
Sent: Thursday, February 12, 2026 3:35 PM
To: smcwg-...@groups.cabforum.org
Subject: [Smcwg-public] Combined ServerAuth+CientAuth for SMTP connections
Dear Group,
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