Use two separate safeguards: require an explicit policy or human approval before an email can be sent, and keep durable per-message send state so retries or concurrent agent calls cannot submit the same logical message twice. If a request times out or returns an ambiguous result, treat its outcome as unknown—not as permission to resend—until you check the application record and provider mailbox.
Why an email can be duplicated—or sent without approval
An AI email workflow has two different failure questions. Authorization asks whether the agent is allowed to send this message to these recipients. Idempotency and reconciliation ask whether this logical message has already been submitted or sent. A human approval step can prevent unwanted mail, but it does not by itself prevent a duplicate submission. A send-state record can suppress duplicates, but it does not establish that the message was appropriate.
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Ambiguous API outcomes make the second problem especially important. Gmail’s API error guidance warns, “You can’t assume that a 200 response means the email was successfully sent.” Microsoft Graph says that 202 Accepted means a request was accepted, not that processing has completed. Neither response should be treated as universal proof of final delivery. Gmail API error guidance; Microsoft Graph sendMail.
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Let the agent draft and revise, but make the application—not the model’s own judgment—the authority that decides whether the side-effecting send operation can run. Define a policy that fits the workflow: for example, require human approval for external recipients, new domains, sensitive content, or every message. If a message is not approved under that policy, the send tool should refuse it.
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- Show the approver the recipient list, subject, complete body, and attachments.
- Bind approval to the exact reviewed message, including its recipients and content version or hash.
- Invalidate approval if the recipient, body, subject, or attachments change; require review of the revised message.
- Restrict recipient classes or domains and cap the number of sends per agent run when those limits suit the task.
- Provide an operator pause or kill switch that blocks new send operations.
These are application-level controls. Creating a provider draft can give reviewers a natural checkpoint, but a draft workflow alone does not require a person to approve it.
Give every logical send a durable identity
Assign an application-generated ID to each intended email action and persist its state. The identity should represent the logical send, not merely an individual API attempt: a retry of the same intended message must resolve to the same record. Store enough information to recognize what was authorized and what happened, such as the recipient set, content version or hash, agent run ID, approval status, provider response, and final observed status.
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At the send boundary, claim or transition that record atomically before calling the provider. Enforce uniqueness so two workers—or two repeated tool calls—cannot both proceed as though they own the same send. The reviewed Gmail and Graph documentation does not promise a general idempotency key for these send operations, so this protection must be designed in the application rather than assumed from the API.
Use explicit states, including an unknown outcome
A practical state machine might be drafted → awaiting_approval → approved → submitting → accepted/unknown → reconciled_sent, with a separate failed state for failures known to have occurred before submission. The labels are an application design, not provider-defined statuses. Their purpose is to prevent uncertainty from silently turning into a resend.
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- Before submission: Confirm the message is approved and that its logical ID has not already been claimed.
- While submitting: Record the attempt and its timestamp before or as the provider call is made.
- After an ambiguous result: Mark the outcome
unknown; do not automatically submit another copy. - After reconciliation: Record whether the provider mailbox shows the message or whether there is reliable evidence the attempt did not complete. If uncertainty remains, hold for operator review.
What to do after a timeout or ambiguous response
- Stop automatic retries for that logical message. A timeout or lost connection after submission may mean the provider received the request even though the agent did not receive a conclusive result.
- Check the application send record. Look up the logical action ID, attempt timestamps, approval record, provider response, and any provider message or draft ID.
- Inspect the provider mailbox or available message state. Reconcile the record against what is visible there instead of relying only on the agent’s last tool result.
- Choose a controlled outcome. If evidence establishes the message was sent, mark it reconciled and do not resend. If a failure is clearly known to have occurred before submission, retry within bounded limits. If the result remains unclear, hold it for human review.
This is a cautious engineering pattern, not a provider-guaranteed exactly-once delivery mechanism. The cited API documentation describes uncertainty in responses; it does not promise that mailbox inspection can resolve every ambiguous case.
Separate safe retries from risky retries
Retry only when the failure is known to be pre-submission or when provider guidance gives a safe recovery path. Use bounded backoff rather than an unending retry loop, and follow the provider’s quota and throttling instructions. A 429 Too Many Requests response is a rate-limit signal, not a reason to retry immediately in a tight loop. A timeout after the send call, by contrast, can leave the result uncertain and should trigger reconciliation first.
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Keep a visible audit trail for the logical action ID, agent run, recipient set, approval record, provider message or draft ID where available, timestamps, response code, and retry reason. That trail helps an operator distinguish repeated agent intent from a repeated API attempt and pause the workflow before more messages are sent.
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Use drafts when review should happen before sending
Microsoft Graph supports creating a message as a draft and sending it in a later operation. This separates composition from the send action and can make review easier to enforce in the application. The separation is a useful workflow boundary, not an approval feature supplied automatically by Graph: the application still needs to prevent the send operation until the required approval state exists.
- Create the message as a draft with Graph’s Create message operation.
- Present the draft’s recipients, subject, body, and attachments for review, and bind approval to that exact version.
- Only after approval, invoke Graph’s Send draft operation and update the application’s send record.
Understand provider responses, quotas, and limits
| Provider detail | What it establishes | What it does not establish |
|---|---|---|
| Gmail API response and errors | Google’s error guidance says a 200 response cannot be assumed to mean the email was successfully sent. It also documents 429 errors, daily sending limits, per-user concurrent-request limits, and that quota errors can take time to surface. See Resolve errors. |
It is not a general exactly-once guarantee or proof of final delivery. Check the current Gmail API usage limits for the applicable Google Cloud project; Google states quota changes take effect May 1, 2026 for applicable projects, and applicability depends on project history. |
Microsoft Graph sendMail |
The endpoint returns 202 Accepted when the request is accepted; Microsoft says this does not mean processing has completed. Delivery is also subject to Exchange Online limitations and throttling. See sendMail. |
Acceptance is not confirmation of completed processing or final delivery, and the endpoint documentation does not promise exactly-once behavior. |
Rate limits and sender rules protect service capacity and deliverability; they do not deduplicate a logical send. Google recommends a consistent sending rate and monitoring. For senders sending more than 5,000 messages per day to Gmail accounts, Google’s sender guidelines require SPF, DKIM, and DMARC; Google recommends keeping the spam rate shown in Postmaster Tools below 0.30%. These are sender and reputation requirements, not duplicate-send controls. See Google Email sender guidelines.
Test the workflow before enabling live sending
Use a test mailbox or controlled recipients. Exercise both repeated tool invocations and the case where a send request reaches the provider but the application experiences a timeout. Verify that the same logical ID cannot be claimed twice, that an uncertain result enters a hold or reconciliation path, and that approval becomes invalid if the reviewed message changes. These checks test the application’s safeguards; they do not prove provider-level exactly-once delivery.
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