Outlook: Invalid signature of mixed HTML/Plaintext E-Mails

Question

while testing new security GPOs, i've found the problem that with E-Mails that contain HTML and Plaintext, Outlook always complains about invalid signatures or tempered messages, even though with the same E-Mails as pure HTML or pure Plaintext, Outlook has no problem with the signature. Can i fix this, or is this a Outlook bug?

This doesn't seem to be a problem with forced conversion, and i cant force use of HTML or Plaintext.

With invalid signatures, i mean that Outlook complains that the signature is invalid or that the message has been tempered with. — Björn Max Jakobsen, Aug 30 '21 at 11:59
Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community, Sep 05 '21 at 02:10

score 0 · Answer 1 · answered Aug 31 '21 at 02:45

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What is your email server? Exchange server?

Does this issue occur to OWA If you use exchange accounts?

In addition, I want to confirm if this issue only occur to the outlook clients applied GPO?

answered Aug 31 '21 at 02:45

Joy Zhang

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We have an exchange server, it happens to all clients. – Björn Max Jakobsen Sep 01 '21 at 05:03
Could you disable the GPO and check if this issue continues? – Joy Zhang Sep 07 '21 at 07:32
It does continue – Björn Max Jakobsen Sep 07 '21 at 09:33

Outlook: Invalid signature of mixed HTML/Plaintext E-Mails

1 Answers1