Email Fixture Lab

Inspect an .eml file without uploading mail or opening its HTML. A small offline beta for people building email workflows, including Cloudflare Email Workers. Independent software by Valen Systems, not affiliated with Cloudflare.

Try the ZIP

Install Node.js 22.13+, unzip, and run from the extracted directory:

node email-fixture-lab.mjs inspect examples/bounce.eml

To inspect your own file, run from the directory containing it:

node /path/to/email-fixture-lab.mjs inspect message.eml

The output describes body sizes, known MIME categories, attachments, nested messages and delivery-status fields. Email addresses become address-1, address-2, etc. Aliases preserve matching addresses within this inspection, including the nested original message; they reset for each file and are not cross-file identities. Exact address case is preserved internally when matching.

Subjects, display names, raw headers, message IDs, filenames, Content-IDs, diagnostic text and bodies are omitted. Unknown MIME types become other. Attachment bytes are never written or executed. Receipts contain input and attachment SHA-256 fingerprints, sizes, relationships and reported status codes: these can still be sensitive. Hashing is not encryption or anonymization. Review a receipt before sharing it.

Related MCP server: DingusMail

Local MCP

{
  "mcpServers": {
    "email-fixture-lab": {
      "command": "node",
      "args": ["/path/to/email-fixture-lab.mjs", "mcp", "--root", "/path/to/fixtures"]
    }
  }
}

The only tool is inspect_email_fixture, with { "file": "bounce.eml" }. Select a relative .eml path beneath the startup root. No directory discovery, URL input, network access, credentials, sending, account setup or file writes are exposed. Concurrent inspections return BUSY rather than queueing. CLI inspection uses the current directory as its root. Keep the root stable: this is not a sandbox against another local process replacing directories during a read.

Beta limits

• Input: 1 MiB, regular files only, no symlinks in the selected relative path.

• MIME: 32 levels and 64 KiB aggregate headers per parsed message.

• Embedded message/rfc822: four levels below the outer message, 16 messages total.

• Across the inspection: 128 attachments and 256 address occurrences.

• Delivery-status part: at most 16 recipient blocks. Only standard action values and numeric enhanced-status codes are emitted. Duplicate/missing/unrecognized fields become null; fieldsValid checks only these extracted fields. actionStatusConsistent compares action and status class.

Exceeded limits fail with fixed error codes, without raw input in diagnostics. The parser is tolerant of malformed MIME; a successful inspection is not RFC conformance, a malware verdict, exhaustive MIME-tree validation or a claim that delivery occurred. Internationalized/global DSN extensions and arbitrary nested containers are outside v0.1. A DSN can be forged; this tool reports what a local fixture says. ZIP, PDF and image attachment contents are not recursively inspected. HTML is parsed as data, never rendered.

CLI exits: 0 inspected, 64 input/parse/limit error, 70 unexpected error. No runtime dependency installation is needed for the ZIP. Windows execution has not been tested.

Source and build

npm ci --ignore-scripts
npm test
npm run build

The build uses the host's zip; package tests use unzip. Source CLI: node src/cli.mjs inspect examples/bounce.eml. Original adapter code is MIT licensed. Bundled dependencies retain their licenses, supplied in licenses/ and THIRD_PARTY_NOTICES.md.

Parser: PostalMime 3.0.0, MIT-0. Its documented MIME/header limits are applied before extracting output. Delivery-status fields follow the core format in RFC 3464. All included messages are synthetic; no private mail code or messages were used.