To test an AI email agent without contacting real people, route the application’s outgoing SMTP mail to a local capture server such as aiosmtpd, bound to localhost. That lets you inspect messages without delivering them. If the test must receive an external verification email, use a separate test inbox on a controlled domain: a local mock SMTP server captures outgoing mail; it does not receive messages sent by a third-party service.
Neither setup alone makes an agent safe. Also restrict network access and credentials, isolate the test environment, and require policy checks before consequential email actions.
What a mock SMTP server does—and does not do
A local SMTP capture server accepts messages from your application and displays them for inspection instead of forwarding them to recipients. It is useful for checking the generated recipient, subject, headers, and body. Django documents this approach with aiosmtpd; the project describes the package as an asyncio implementation of SMTP and LMTP, with a command-line server suitable for testing SMTP clients. Its stable documentation identifies version 1.4.6, so check the current instructions and compatibility for your environment before installing.
A capture server does not establish that a message can be delivered by a real provider, nor can it receive an OTP or confirmation link sent by an external signup service. Those tests need a controlled receiving inbox or provider integration.
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Run a local SMTP capture server
Django’s documentation gives this minimal aiosmtpd setup. It listens on loopback at port 8025 and prints email headers and body to standard output rather than delivering the message.
-
Install the package:
python -m pip install "aiosmtpd >= 1.4.5" -
Start the listener in a terminal:
python -m aiosmtpd -n -l localhost:8025 -
Configure the application’s email backend to use SMTP host
localhostand port8025. For Django, use the SMTP email backend and set its host and port to those values. -
Trigger the agent’s email action and inspect the output in the listener terminal. Confirm the intended recipient, subject, headers, and body; test that disallowed recipients are rejected by your application’s policy.
Keep the listener on loopback for ordinary local development. Binding it to a wider interface exposes the test service to other machines and should only be done for a deliberate, isolated test topology.
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These commands and behavior are documented in Django’s email documentation; consult the version of Django you actually deploy. The aiosmtpd stable documentation is the primary reference for current package options.
When you need a test inbox instead
If an external service sends a verification email during a test, provide a fresh address on a development domain with a controlled inbox. Do not give the agent access to a personal or production mailbox just to retrieve a code.
SMTP.dev’s guide describes a workflow that creates a catch-all on a development domain, generates a unique address for each run, and retrieves messages by matching the actual recipient address. It describes polling for short-lived runs and server-sent events (SSE) for a long-running agent. The guide also says its sandbox domain accepts external inbound messages, while outbound messages from the sandbox are limited to accounts inside that domain. Those are service-specific claims; verify the exact inbound, outbound, retention, and access rules of whichever inbox provider you choose.
Give the agent only the mailbox capability the test needs: ideally a per-run address and restricted access to the test domain. SMTP.dev documents API-key authentication through an X-API-KEY header. Treat that token as a secret, not as prompt text or generated source code; keep it outside agent-readable files where possible, or have a trusted service broker access.
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See SMTP.dev’s AI-agent inbox guide for its documented workflow.
Build security boundaries around the mail route
A mock SMTP server is one control at the mail boundary, not a complete sandbox. An agent may have other routes to external systems if its environment permits them. OpenAI’s sandbox guidance warns that agent-generated code can read credentials available to its environment and recommends isolating workloads, restricting outbound traffic to approved endpoints, and brokering third-party credentials through a trusted proxy or external server.
- Compute: run the workload in isolated compute and separate environments that should not share data.
- Network: allow outbound connections only to required, approved endpoints. Make sure the agent cannot bypass the mock route by connecting directly to a real SMTP provider.
- Credentials: keep production mail credentials out of agent-readable files, environment variables, source code, and logs. If a test needs a third-party credential, use a trusted broker or proxy and narrowly restrict its destinations.
- Mail: use local capture for outbound-only tests, or a separate test domain and inbox for external inbound flows. Verify the chosen service’s actual delivery boundary.
- Actions: enforce an application-level policy or require confirmation before a consequential real-world send.
OpenAI’s sandbox security guidance covers isolation, egress, and credential handling. These controls reduce exposure but do not replace application-level authorization.
Treat incoming email as untrusted input
A verification inbox can also expose the agent to hostile or irrelevant message content. OpenAI defines prompt injection as a third party misleading a model by inserting malicious instructions into its context. An email the agent reads is third-party content, not a trusted instruction source. Limit mailbox access to the task, give the agent specific rather than broad authority, and review consequential actions such as sending mail. OpenAI notes that such measures are defense in depth and may not prevent every injection.
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For email construction, validate untrusted fields and recipient handling as well. Django describes CRLF injection as a way to add headers using carriage-return and line-feed characters. Its modern email message path rejects such injection with ValueError; custom backends that bypass that path, or use the legacy compat32 policy, must provide their own CRLF protection. A mock server lets you inspect output, but it does not substitute for input validation and safe header construction.
Read OpenAI’s prompt-injection guidance and Django’s email security notes for the relevant safeguards.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.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 | External delivery or receipt of mail from third-party services |
| Deterministic in-memory tests | Application-owned orchestration, tool calls, retries, and policy checks | Real provider or network-protocol behavior |
| Hosted test inbox on a separate domain | Flows that need externally delivered OTPs or confirmation links readable through an API | Behavior beyond the provider’s documented domain, credential, and delivery limits |
| Controlled real-provider integration | Provider and delivery behavior that local tests cannot represent | Safe execution unless credentials, recipients, and network access are bounded |
The OpenAI Agents SDK documentation distinguishes deterministic in-memory tests from external-provider and network-protocol testing. Its in-memory utilities can record normalized SDK-owned interactions without making model, sandbox-provider, or Realtime API requests. Use them to test your application’s orchestration, then add a controlled integration only for behavior that depends on a real provider or protocol.
See the OpenAI Agents SDK testing documentation for the in-memory testing boundary. No single approach is universally best: choose based on whether you need outbound capture, inbound reception, deterministic policy checks, or real external behavior.
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Use a staged test sequence
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Test orchestration deterministically. Mock the model, mail tool, or transport as appropriate. Assert the intended recipients and message content, and verify that unauthorized send actions are not invoked. Avoid unintentionally sending traces or test content to external systems.
-
Capture outgoing mail locally. Point the application at the loopback
aiosmtpdlistener. Inspect the emitted message and test recipient allowlisting and rejection behavior. -
Add inbound end-to-end testing only if needed. Create a fresh test address for the run, submit it to the signup flow, and retrieve a message matching that recipient. Check expected sender and content before extracting a code or link; do not treat arbitrary body text as instructions.
-
Check network and secrets. Confirm the sandbox cannot contact real mail-delivery endpoints or production inbox APIs except through explicitly approved services. Keep keys outside agent-readable code where possible.
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Run a bounded provider integration when the claim depends on it. Use synthetic accounts and restricted destinations to exercise real-provider behavior that the local mock cannot model.
This separation makes test results more meaningful: a captured message demonstrates what reached the local SMTP boundary, while only a controlled external integration can exercise external delivery.
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