This message indicates that your keychain contains an invalid certificate for the sender’s email address.
- Quit Mail if it's open.
- Open Keychain Access, located in the Utilities folder, in the Applications folder.
- Click the “Login” keychain in the left column.
- Locate the certificate with the same name as the email address displayed in the message that appeared when the email did not send.
- Remove the certificate from the keychain by pressing the Delete key.
- Open Mail.
- Attempt to send the email message again.
Note: This issue doesn't occur with Mail messages that have signatures, such as your name.
What is a “signed message”?
If you have a personal certificate on your computer, you can send secure signed messages (including the body of the message and any attachments) to anyone using Mail. A personal certificate is an electronic document that associates your digital identity with other information, such as your name or email address. If you want to send and receive signed and encrypted messages, you need a personal certificate.
Signed messages let your recipients verify your identity as the sender, and provide assurance that the message has not been tampered with in transit. A Signed icon (a checkmark) in the email header indicates your personal certificate is installed in Keychain Access.