Digitally Signed Email Message
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What is a P7S file?
P7S files are found in email messages that include an S/MIME digital signature. They indicate the message has been secured and can be read only by the intended recipient. P7S files verify that the email is from who it claims to be from and that the email has not been modified in transit.
Most email applications, such as Microsoft Outlook, Apple Mail, and Mozilla Thunderbird, support digital signatures. However, if an email program does not handle digital signatures, a P7S file may appear as an attachment (smime.p7s) to a digitally signed email message.
NOTE: Email programs that use P7S files use PKCS (Public-Key Cryptography Standard) #7 to sign email messages.
How to open a P7S file
You can open a P7S file in Microsoft Outlook, Apple Mail, and other email clients that support S/MIME digital signatures. To do so, you must be an intended recipient of the email, you must be listed in the sender's address list, and you must have installed the S/MIME control. (Typically, Microsoft Outlook warns users if they are about to send a P7S email to a user who will not be able to open the email.)