Signature rules
Signature rules allow the application to automatically append predefined user signatures to emails based on specific conditions. This feature ensures consistent branding and compliance with company policies.

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Signature rules leave signed and encrypted messages untouched. Inserting a signature would break the sender's digital signature, and the text of an encrypted message cannot be changed. The processing trace shows Cannot add signature to signed message. (or encrypted). Use the Encrypted and Signed values of the Message type condition to find these messages.