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Based on my research, unfortunately, it appears that creating a transport rule in Exchange 2019 does not enable extracting the public key from the email sender's certificate and saving it to a specific location or adding it to a centralized repository. Exchange Transport rules are mainly used to manage email flow based on specified conditions, usually with the following operations:
Besides, you could refer to Mail flow rule actions in Exchange Server | Microsoft Learn to view common mail flow rule actions.
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