Hey 1950Barb,
Welcome to Apple Support Communities.
Is this happening with any emails your attempt to send or is it happening with one specific email? Is it an email message that has a digital signature or encrypted? If so, the steps below should help.
1. In the Mail app
on your Mac, choose File > New Message.
2. Move the pointer over the From field, click the pop-up menu that appears, then choose the account for which you have a personal certificate in your keychain.
A signed icon (containing a checkmark) is shown in the message header and indicates your message will be signed when you send it.
3. Address the message to recipients.
You can also see the steps and more information about this enhanced security feature here: Sign or encrypt emails in Mail on Mac.
Take care.