Messages by using S/MIME digtal singature for extenal users show as attachment MIME.p7s file

DanielDanielHashanth-9263 1 Reputation point
2020-12-07T11:12:24.827+00:00

External user receive an email with a smime.p7s file attachment when send the email with digital signature with external users. Internal users show the an email signed with a trusted certificate can be found by clicking on the red ribbon. However, questions do arise if the signed messages arrive in other mail hosts, in which the SMIME.p7s file is seen as an attachment.how we can fix this issue?

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  1. Yuki Sun-MSFT 40,871 Reputation points
    2020-12-08T04:03:09.483+00:00

    Hi DanielHashanth,

    However, questions do arise if the signed messages arrive in other mail hosts, in which the SMIME.p7s file is seen as an attachment.

    Based on my research, this could be an expected behavior when the signed messages arrive in mail hosts which don’t support digital signatures. Given this, you may tell the external recipients that it's safe to trust the email and there is no need to worry.

    Here are some relevant links for your reference:
    What is a smime.p7s email attachment? (See the "Other mail hosts" part)
    What is a smime.p7s File and How to Open It?
    Please Note: Since the web sites are not hosted by Microsoft, the links may change without notice. Microsoft does not guarantee the accuracy of this information.

    If the response is helpful, please click "Accept Answer" and upvote it.
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