To test an AI email agent without emailing real people, route its outgoing mail to a local SMTP capture server and block the agent from reaching real mail providers. A mock SMTP server lets you inspect messages the application tries to send; it does not receive verification emails sent by external services. For those tests, use a separate, controlled test inbox. These are distinct capabilities, and neither one alone makes an agent safe.
What a mock SMTP server does—and does not do
A local mock SMTP server accepts outgoing messages from your application and displays them for inspection rather than delivering them to their intended recipients. It is useful for checking whether an agent chose the right recipient, subject, headers, and body before any real delivery is possible.
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Django documents using aiosmtpd for this purpose. The example listens on localhost:8025 and prints message contents to standard output. The aiosmtpd documentation describes the project as an asyncio implementation of SMTP and LMTP, with a command-line server and a library that can be used to test SMTP clients. Its stable documentation identifies version 1.4.6; check the project’s current instructions and compatibility for your environment.
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Run a local SMTP capture server
For a local development test, install aiosmtpd and start its listener on loopback:
python -m pip install "aiosmtpd >= 1.4.5"
python -m aiosmtpd -n -l localhost:8025Leave that process running while you test. In the application’s email configuration, set the SMTP host to localhost and the port to 8025. Django’s documented example prints the email headers and body in the server’s terminal rather than sending the message onward. Keep the listener bound to loopback unless a specific, isolated test topology requires another arrangement.
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Send a test through the same application path the agent uses, then inspect the captured recipient, subject, headers, and body. Include cases where the recipient should be allowed and where it should be rejected. This tests the application’s message construction and its use of the configured SMTP transport; it does not demonstrate what a provider would do with the message.
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Use a test inbox when the agent must receive external email
If a signup or account-verification flow sends a one-time code or link from an external service, the test needs a receiving inbox in addition to—or instead of—the local outgoing capture server. SMTP.dev’s AI-agent guide describes creating a catch-all on a development domain, generating a fresh address for each run, and finding messages by matching the actual recipient. For short-lived runs, the guide suggests polling; for a long-running agent, it describes server-sent events (SSE).
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The SMTP.dev guide says its sandbox domain accepts inbound messages from external senders, while outbound messages from that sandbox are delivered only to accounts inside the same domain. That is a claim about this service, not a general property of test inboxes. Check the actual delivery rules, retention, API scopes, and outbound behavior of whichever provider you choose.
- Create a synthetic test account and a unique address for the test run on a domain you control or a suitable hosted test domain.
- Submit that address to the external signup or verification flow.
- Retrieve only the message matching the expected recipient, then check its sender and expected content before extracting the code or link.
- Keep the agent from treating arbitrary instructions in the message body as commands, and restrict its access to the mailbox and test domain.
Do not grant a development agent access to a personal or production mailbox just to complete a test. SMTP.dev documents API-key authentication through an X-API-KEY header. Treat that key as a sensitive capability: scope it as narrowly as the service allows and keep it out of prompts and generated code where possible.
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Build security boundaries around the mail route
A mock SMTP listener changes where configured mail goes; it does not isolate the agent or remove credentials and network paths already available to it. OpenAI’s sandboxing guidance warns that agent-generated code can read credentials available in its environment. It recommends isolating workloads, restricting outbound network access to approved endpoints, keeping application keys outside the environment, and brokering third-party credentials through a trusted proxy or external server.
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- Network: Allow outbound connections only to required, approved endpoints. In particular, prevent direct access to real SMTP providers when the test is supposed to use the local capture server.
- Credentials: Keep production mail credentials out of agent-readable files, environment variables, source code, and logs. Use a trusted broker or proxy for any necessary third-party credentials.
- Mail: Use local capture for outbound-only tests, and a separate test domain or inbox only when external inbound mail is required. Confirm the provider’s actual boundaries.
- Actions: Enforce an application-level policy or confirmation step before consequential external actions, such as sending a real email.
Incoming mail is also untrusted input. OpenAI’s prompt-injection guidance defines prompt injection as third-party content that misleads a model by inserting malicious instructions into its context. An email the agent is asked to read can carry such content. Limit mailbox access to what the task needs, give the agent specific instructions rather than broad authority, and review consequential actions. These are defense-in-depth measures, not a guarantee that every injection will be prevented.
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Test message construction as well as agent policy. Django’s email documentation describes header injection, including CRLF characters used to add unintended headers. It says Django’s modern email message path rejects CRLF injection with ValueError; custom backends that bypass that path or use the legacy compat32 policy must provide their own CRLF protection. A mock SMTP server does not replace input validation or header-safety checks.
Choose the test boundary that matches the question
| Approach | Best for | What it does not establish |
|---|---|---|
| Local mock SMTP server (aiosmtpd) | Capturing application-generated outgoing messages and inspecting headers and body during development. | External delivery or receipt of messages sent by third-party services. |
| Deterministic in-memory tests | Application-owned orchestration, tool calls, retries, and policy checks. | Behavior of a real provider or network protocol. |
| Hosted test inbox on a separate domain | End-to-end flows that need externally delivered codes, confirmation links, or other test mail readable through an API. | Service behavior beyond its documented domain, scopes, retention, credentials, and outbound limits. |
| Controlled real-provider integration test | Provider and delivery behavior that a local mock cannot represent. | Safe operation without bounded recipients, carefully controlled credentials, and restricted network access. |
The OpenAI Agents SDK testing documentation distinguishes deterministic in-memory testing from tests that exercise external providers or protocols. Its in-memory utilities make no model, sandbox-provider, or Realtime API requests and can record normalized interactions owned by the SDK. That makes them useful for checking application-owned behavior without making external calls, but not a substitute for a provider integration test.
Choose based on whether you need to inspect outgoing mail or receive external mail, whether the test must be local or hosted, how addresses and credentials are isolated, and whether deterministic behavior or real external delivery is the point. No single option covers all of those needs.
Quick Recap
Use a staged test sequence
- Test orchestration first. Mock the model, mail tool, or SMTP transport as appropriate. Assert recipients and message content, and verify the agent cannot invoke an unauthorized send action. Keep tests deterministic and avoid sending traces or test content to external systems unintentionally. The Agents SDK’s in-memory utilities can record SDK-owned interactions without making model, sandbox-provider, or Realtime API requests.
- Capture outgoing messages locally. Point the application’s SMTP configuration to the loopback aiosmtpd listener. Inspect the captured headers and body, and test that destinations outside the allowed test setup are rejected.
- Add an inbound test inbox only if needed. Generate a fresh address for the run, submit it to the verification flow, and retrieve a message by its actual recipient. Check the expected sender and content before using a code or link; do not treat other email-body text as trusted instruction.
- Check isolation and secrets. Verify that the agent cannot reach real mail delivery endpoints or production inbox APIs except through explicitly approved services. Keep production keys outside agent-readable code and use a trusted broker when third-party credentials are required.
- Run a controlled integration test for external behavior. Use synthetic accounts and bounded destinations when the question depends on a real provider or network protocol. A local mock and deterministic in-memory test cannot establish those external behaviors.
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