It is wrong to assume that written settlement communications you have with opposing counsel are privileged. It is not. So, your attempt to claim a privilege that does not exist is futile. However, nothing stated in settlement discussions (written or otherwise) can be admitted to prove liability or to prove the invalidity of plaintiff’s claim.
If you chose to be cautions, you can label any written communication which contains settlement terms or discussions (via email or otherwise): Confidential Settlement Negotiation (or similar), Evidence Code sections 1152, 1154. That will highlight the fact that you view the communication inadmissible to prove liability. Still the communication could be admitted for other purposes, either in total or part.
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Mr. Daymude is correct. If your communications are an attempt to settle a matter, mark the email clearly as "Settlement Negotiations" so that you and the court later can easily know that the particular email was part of the negotiations. Then move to exclude any of the communication at that time.
No agreement is required to invoke the right to exclude the settlement communications at the time of trial.
Good luck to you.
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There would be no privilege, but settlement discussions are not admissible at trial, so just label your emails as settlement discussions and then if opposing counsel tries to use it against you, you can object.