A digital signature isn't the same as a signature you routinely include with an outgoing message. Anyone can copy an email signature, which essentially is a customizable closing salutation. But a digitally signed message can come only from the owner of the digital ID used to sign it. The digital ID provides verification of the sender's authenticity, thereby helping to prevent message tampering.
Digitally sign a single message
1- In the message, on the Options tab, in the Permission group, click Sign Message.
2- Compose your message, and then send it.
Digitally sign all messages
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On the File tab, click Options >Trust Center.
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Under Microsoft Outlook Trust Center, click Trust Center Settings.
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On the Email Security tab, under Encrypted Mail, select the Add digital signature to outgoing messages check box.
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If available, you can select one of the following options:
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If you want recipients who don't have S/MIME security to be able to read the message, select the Send clear text signed message when sending signed messages check box. By default, this check box is selected.
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To verify that your digitally signed message was received unaltered by the intended recipients, select the Request S/MIME receipt for all S/MIME signed messages check box. You can request notification telling you who opened the message and when it was opened, When you send a message that uses an S/MIME return receipt request, this verification information is returned as a message sent to your Inbox.
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To change additional settings, such as choosing between multiple certificates to use, click Settings.
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Click OK on each open dialog box.