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 same encryption type.

This error can occur in a topology where an enterprise has federated an AD FS server with Azure Active Directory for signing in to Microsoft 365, and also federated the AD FS server with another non-Microsoft federation server such as Shibboleth.

You can use a digital signature for many of the same reasons that you might sign a paper document. A digital signature is used to authenticate digital information — such as form templates, forms, e-mail messages, and documents — by using computer cryptography.

You can use encryption and digital signatures in new Outlook, classic Outlook, and Outlook on the web for work or school accounts to increase the security of messages. Encrypting an email message in Outlook means it's converted from readable plain text into scrambled cipher text.

Open the digitally signed message. Look at the Signed By status line to check the email address of the person who signed the message. It isn't enough to check the email address in the From line—you want to verify who actually signed the message, not only who sent the message.

The Signature Details dialog box displays certificate information such as the signer’s name in the Signing as box, and who issued the certificate. Open the file that contains the certificate you want to view.

The Outlook Team fixed the root cause issue that was preventing signature sync. Initially the fix is available in the Beta Channel and Current Channel Preview with Version 2411 (Build 18205.15010).

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If you cannot sign in to your OneDrive sync app, or the sync app says OneDrive is not connected, follow these steps to get signed in again. If you see a gray OneDrive icon with a line through it, this means you are not signed in.

You can change the default signature that automatically appends to every email you send. You can also update an existing email signature. Here's how: On the Outlook menu, click Settings. Under Email, click Signatures.