Emails with digital signatures being converted to attachments

Kenny Stern 371 Reputation points
2020-12-24T13:23:10.037+00:00

Since migrating to M365 emails from a particular company that uses digital signatures are being received with no body and the entire email as an attachment. The emails contain a digital signature, which Outlook says is invalid.
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When I click on that it brings this up.
51080-image.png

The certificate is not expired and the CA is GlobalSign. The same email sent to my Gmail account does not report the signature as invalid. So something specific to 365 is causing the body of these emails to get stripped.
I followed the steps in this article, https://techcommunity.microsoft.com/t5/exchange-team-blog/how-to-configure-s-mime-in-office-365/ba-p/584516, to export the Root and Intermediate certs to an SST and then importing the SST into M365. But it didn't help and email body is still being stripped and emails are still being received as attachments.
We have users who receive a lot of email from this company and having to open an attachment for every email is not efficient.

Is there something I can do to prevent this behavior?

Thanks,
Kenny

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  1. Andy David - MVP 142.7K Reputation points MVP
    2020-12-24T15:29:34.54+00:00

    Ok, I would suggest opening a ticket with Microsoft 365 support. There may be a bigger issue going on.