Update Mailbox Signature

PATCH https://app.mailivery.io/api/v1/campaigns/{campaignId}/signature

Create or update the email signature for a single mailbox (campaign).

• If no signature exists, Mailivery will create one.
• If a signature already exists, only the fields you provide will be updated.

Use this endpoint to keep mailbox signatures in sync with your CRM, HR system, or onboarding flows.