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Your digital signature can be imported to Microsoft 365® easily by following these instructions:

Microsoft 365 subscriber on build 16.19.18110915 and higher

  1. If you don't see the Sign/Encrypt Message button, you might not have a digital ID configured to digitally sign messages and you need to do the following to install a digital signature.
      a. On the File menu, click Options > Trust Center.
      b. Under Microsoft Outlook Trust Center, click Trust Center Settings > Email 

      c. Click Import/Export to import a digital ID from a file on your computer.
      d. If you have both a signing and an encryption certificate you will import both
  2. A digital signature on an e-mail message helps the recipient verify that you are the authentic sender and not an impostor. To use digital signatures, both the sender and recipient must have a mail application that supports the S/MIME standard. Outlook supports the S/MIME standard.

Microsoft 365 subscriber on build and higher

  1. In an email message, choose Options
      a. Select both the Sign and Encrypt buttons.
      b. Pick the encryption option that has the restrictions you'd like to enforce,
          such as Do Not Forward or Encrypt-Only.

Note: Microsoft 365 Message Encryption is part of the 0365 E3 license
Additionally, the Encrypt-Only feature (the option under the Encrypt button) is only enabled for subscribers(Office ProPlus users) that also use Exchange Online.