Mail wants to sign using key "privately" in your keychain.

Hello 15-Pirates,

We have a great support article that might help with this. Please take a look at it here: If Mail on your Mac keeps asking for your password - Apple Support. Follow this guidance:

"Why is Mail asking for your password?

Mail asks for your password when it can connect successfully to your email provider, but can't connect to your email account because your email provider isn't accepting your email password.

Your email provider may reject your password for several reasons, including:

• You're not using your current email password. If you updated the password on another device, make sure that you're using the same password on your Mac.
• Your email provider needs you to update or reset your password for security reasons. For example, if you're using Google’s two-step verification, your Gmail account in Mail requires the appropriate app-specific password.
• Your email provider deactivated or suspended your account. Contact your email provider for help.

Check your password in Internet Accounts preferences

Depending on the type of email account you're using, you might be able to correct the password as follows:

1. Quit Mail. If it doesn't quit, you can force it to quit.
2. Choose Apple menu  > System Settings (or System Preferences), then click Internet Accounts.
3. Click your mail account.
4. Click the Details button, if present.
5. If you see a password field for your account, delete the password and type the correct password.
6. Close the settings window, then open Mail and try using your account again.

If Mail still asks for your password, the password is probably incorrect. Your email provider can help you verify your email settings and make sure that you're using the correct password.

Test using webmail

Many email providers offer webmail, which is a way to use email with a web browser such as Safari, instead of with an email app such as Mail. For example, if you're using an iCloud Mail account, you can use iCloud.com to send and receive email.

Because the password that you use with Mail is usually the same password that you use with webmail, try signing in to webmail with that password. If it doesn't work there either, your password is probably incorrect. Your email provider can help you verify your email settings and make sure that you're using the correct password for the Mail app.

Remove your account, then add it back

If no other solutions work, try setting up your email account again. To do that, follow these steps:

1. Remove your email account in Mail.
2. Add your email account in Mail again.

Learn more

• If you're using Apple's iCloud Mail, learn how to resolve issues with iCloud Mail. The password that you use with iCloud Mail is your Apple ID password. Learn what to do if you forgot your Apple ID passwordor want to change your Apple ID password.
• If you forgot or want to change the email password for a different email service, contact the email provider or request a password reset from their website."

We hope this helps. Let us know if we can assist with anything else at all. Take care.