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Use cases → Email operations

Inject custom footers into outbound emails via API

Add or update standard disclaimers, signatures, or compliance footers to users’ sent mail by rewriting messages with domain-wide delegation.

Who it is forIT automation engineers responsible for compliance or branding across user email.
APIsGmail API
Typical scopesgmail.modify

The problem

Legal or branding policies often require a footer on every outbound email. Relying on user settings is unreliable, and updating signatures manually does not catch programmatic sends or mobile clients.

How it works

  1. Use a service account with domain-wide delegation and the 'gmail.modify' scope.
  2. Set up a watch on the 'Sent Mail' label to detect new outbound messages.
  3. Fetch recent sent messages for each user on a schedule.
  4. Rewrite the message body to append or update the footer, then use the 'messages.modify' endpoint to patch the message in place.

What changes

Every sent email, regardless of client or sending method, carries the correct footer as required by policy.

The trap in this one. Gmail’s API does not guarantee immediate consistency between send and modify: messages can be processed before the footer is injected, especially for rapid-fire sends or large accounts, leading to emails being delivered without the required footer. There is also no atomic operation—the message may be briefly visible or delivered without the injected content if the modify call is delayed or fails, with no retry guarantee.

Questions people ask

Can this add footers to all outbound mail, including mobile and API sends?

Yes, as long as the message appears in the user’s Sent Mail and has not been delivered yet. However, race conditions mean some messages may escape modification.

Does modifying a sent message recall or re-send it?

No, the Gmail API only updates the stored copy in the user’s mailbox. If the message has already been delivered, recipients see the original version.

Want this built?

This is a pattern we run in production. We will set up the delegation and build this on top of it — $500 per hour, most of it working the same day.

Talk to us Or read the setup guide

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