If a digital signature isn't valid, there can be many causes. For example, the sender's certificate may have expired, it may have been revoked by the certificate authority (CA), or the server that verifies the certificate might be unavailable.

A digital signature is an electronic, encrypted, stamp of authentication on digital information such as email messages, macros, or electronic documents. A signature confirms that the information originated from the signer and has not been altered.

If you want to insert a digital signature into an email, you need to install a digital ID (certificate), which is verified by a third party. Before you can send and receive messages with encryption or digital signatures, or digital IDs, please follow the instructions to set up encryption in Outlook.

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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A digital signature is an electronic, encrypted, stamp of authentication on digital information such as email messages, macros, or electronic documents. A signature confirms that the information originated from the signer and has not been altered.

When you receive an external email with attachments and a digital signature, you could see the following error message: "There are problems with the signature. Click the signature button for details." This issue occurs because the attachment filtering does not process the email correctly.

To report issues or provide feedback, simply go to Help > Contact Support in Outlook for Mac. For more information, see Contact Support in Outlook for Mac. Have a feature request? To suggest new features in Outlook for Mac, go to Help > Give Feedback. We're listening.

Whether it’s a new job title (congratulations!) or a phone number change, make sure you keep your email message signature up-to-date. If you're using new Outlook, for more information about email signatures, or if you haven’t created one yet, see Add a signature to messages.

When typing an email and pressing the Enter key letters from your signature appear vertically. This issue reproduces when the Outlook Signature contains 2 consecutive <br> tags.