Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add UnboundCompute/security-agent-skills --skill testing-smtp-smuggling-and-email-spoofinggit clone --depth 1 https://github.com/UnboundCompute/security-agent-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/unboundcompute/security-agent-skills/testing-smtp-smuggling-and-email-spoofing)<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/testing-smtp-smuggling-and-email-spoofing"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/testing-smtp-smuggling-and-email-spoofing/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/testing-smtp-smuggling-and-email-spoofing"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/testing-smtp-smuggling-and-email-spoofing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00162 | $0.01899 |
| Opus 5.5 | $0.00065 | $0.00760 |
| Sonnet 5.5 | $0.00032 | $0.00380 |
| Haiku 4.5 | $0.00016 | $0.00190 |
Grade A, and why
testing-smtp-smuggling-and-email-spoofing scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 135 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Testing SMTP smuggling and email spoofing: authenticated, and still forged
Email authentication is a stack of three records that only stops spoofing if all three are present, aligned, and enforced - and even then only if every server on the delivery path agrees where one message ends and the next begins. The failures are quiet: a policy published but set to take no action, a signature that validates a domain that is not the visible sender, a subdomain with no policy, and the boundary desync where two servers parse the end-of-data sequence differently, letting a forged message ride inside a legitimate one and pass every check. You find them by testing what actually gets delivered and trusted, not by reading the records and assuming they hold.
When to use
- You are auditing a domain's email authentication or a mail server's message handling.
- The domain publishes sender-authentication records, or receives mail through more than one hop.
- You can send test mail into the path and observe what is accepted and how it is judged.
Scope check
Test mail authentication and delivery only for domains and servers you own or are authorized to assess, sending only to mailboxes you control. Spoofed mail to third parties is out of scope and often unlawful. If you can't name the authorization, stop.
The loop
-
Map the mail path and the records. Identify the servers a message crosses inbound and outbound, and retrieve the domain's sender-authentication records: the authorized-senders policy, the signing setup, and the alignment-and-reporting policy. Note the published enforcement action and whether it is set to reject, quarantine, or merely observe. The path and the records together define what should stop a forgery.
-
Check enforcement, not just presence. A record that exists but sets its action to none, or a reporting policy in monitor mode, authenticates nothing in practice. Confirm the policy instructs receivers to actually reject or quarantine failures, and that receivers on the path honor it. Present-but-permissive is the most common gap.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago First seen · 135 lines · 162 tokens per session scan A 017678ad065e
testing-smtp-smuggling-and-email-spoofing is a skill published in the GitHub repository UnboundCompute/security-agent-skills (5 stars, last pushed 1mo ago), licensed MIT. It adds 162 tokens to every session and 1,899 once invoked, about $0.0006 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-05.
Other skills, from other repositories
prodcheck-review
Review this codebase against the prodcheck pre-production checklists — security, performance, scale, integrations, post-launch readiness and people. Use when asked to check whether a project is ready to ship, to audit an area before launch, or to work through a specific checklist. Produces evidence with file:line…
integrate-arcjet-guard-genkit
Integrate Arcjet security into a Genkit JS agent using @arcjet/guard — wrap ai.defineTool, put guardMiddleware on generate({ use }) for unwrapped / MCP / filesystem tools, and read a caller-owned id from generate({ context }). Use when asked to add Arcjet to genkit, rate limit its tools, screen inbound messages, or…
integrate-arcjet-guard-tanstack-ai
Integrate Arcjet security into a TanStack AI chat() app using @arcjet/guard — put guardMiddleware first on chat({ middleware }) so onBeforeToolCall gates tools, and read a caller-owned id from chat({ context }). Use when asked to add Arcjet to TanStack AI, rate limit its tools, screen inbound messages, or block prompt…
integrate-arcjet-guard-claude-managed-agents
Integrate Arcjet security into Claude Managed Agents (hosted REST+SSE, beta managed-agents-2026-04-01) using @arcjet/guard — screen user.message / initialevents before sessions.events.send, and gate custom tools on agent.customtooluse. Use when asked to add Arcjet to Claude Managed Agents, rate limit custom tools, or…
linear-claude-skill
Manage Linear issues, projects, and teams.
varlock-claude-skill
Secure environment variable management ensuring secrets are never exposed in Claude sessions, terminals, logs, or git commits.