md-to-eml

type	skill
lifecycle	stable
inheritance	inheritable
name	md-to-eml
description	Convert Markdown to RFC 5322 email (.eml) with inline CSS and CID images
tier	standard
applyTo	**/*eml*
currency	"2026-04-22T00:00:00.000Z"
lastReviewed	"2026-04-30T00:00:00.000Z"

Markdown to Email Conversion

Write in Markdown, send as professional email — works in any email client

Staleness Watch: See EXTERNAL-API-REGISTRY.md for source URLs and recheck cadence

Convert Markdown documents with YAML frontmatter into RFC 5322-compliant .eml files ready for governance, newsletter, and stakeholder communication workflows.

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When to Use

• Sending formatted content via email clients (Outlook, Thunderbird, etc.)
• Newsletter or governance communication from Markdown sources
• Generating test emails for review before batch sending
• Converting documentation into distributable email format
• Creating email templates with consistent branding
• Stakeholder updates with embedded charts or diagrams

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Supported Formatting

Format	Status	Notes
Headings	✅	H1-H6 with inline styles
Bold/Italic	✅	Standard emphasis
Links	✅	Clickable hyperlinks
Images	✅	CID embedded as attachments
Code blocks	✅	Monospace, gray background
Inline code	✅	Highlighted
Tables	✅	HTML tables with borders
Blockquotes	✅	Indented with border
Lists	✅	Ordered and unordered
Mermaid diagrams	⚠️	Table fallback (no JS in email)
Emoji	✅	Unicode preserved
Horizontal rules	✅	Styled dividers

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Key Features

Feature	Details
YAML frontmatter	Maps to RFC 5322 headers (To, From, Subject, CC, Reply-To)
Email-safe HTML	Inline CSS with table-based layout (no <style> blocks)
Mermaid fallback	Diagrams converted to ASCII table representation (email-safe)
CID images	Local images embedded as base64 multipart MIME attachments
Emoji preservation	Subject and body emoji render correctly across clients
Test mode	--test flag overrides recipients for safe preview

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Usage

# Basic conversion
node .github/muscles/md-to-eml.cjs newsletter.md

# With test recipient override (safe preview)
node .github/muscles/md-to-eml.cjs newsletter.md --test --test-to [e-mail]

# Embed images as CID attachments
node .github/muscles/md-to-eml.cjs update.md --inline-images

# Debug mode (saves intermediate HTML)
node .github/muscles/md-to-eml.cjs update.md --debug

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Options Reference

Option	Default	Description
--test	off	Override recipients for safe preview
--test-to ADDRESS	frontmatter from	Custom test recipient email
--inline-images	off	Embed images as base64 CID attachments
--debug	off	Save intermediate HTML for inspection

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Frontmatter Format

The YAML frontmatter maps directly to RFC 5322 email headers:

---
to: [e-mail]
from: [e-mail]
subject: 📊 Weekly Update - Sprint 42
cc: [e-mail]
reply-to: [e-mail]
---

Field	RFC 5322 Header	Required	Notes
to	To	✅	Primary recipient(s), comma-separated
from	From	✅	Sender address
subject	Subject	✅	Supports emoji
cc	Cc	❌	Carbon copy recipients
reply-to	Reply-To	❌	Reply address if different from From

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Email Client Compatibility

Client	HTML	Images	Tables	Emoji
Outlook (Windows)	✅	✅ CID	✅	✅
Outlook (Mac)	✅	✅ CID	✅	✅
Gmail (Web)	✅	✅ CID	✅	✅
Apple Mail	✅	✅ CID	✅	✅
Thunderbird	✅	✅ CID	✅	✅
Mobile (iOS/Android)	✅	⚠️ varies	✅	✅

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Test Mode Workflow

1. Write your newsletter/update in Markdown with frontmatter
2. Convert with --test --test-to [e-mail]
3. Open the .eml file in your email client
4. Review formatting, images, links
5. Convert again without --test for production
6. Send via your email client or automation

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Mermaid Diagrams in Email

Email clients cannot execute JavaScript, so Mermaid diagrams are converted to text-based table representations:

┌─────────────────┐
│ Original Mermaid │ → Table Fallback
└─────────────────┘

flowchart LR    →   | Step | Description |
  A --> B           | A    | Start       |
  B --> C           | B    | Process     |
                    | C    | End         |

For high-fidelity diagrams, pre-render to PNG and include as images.

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Troubleshooting

Problem	Cause	Solution
"pandoc not found"	pandoc not installed	winget install pandoc
Images not showing	CID not supported	Use --inline-images flag
Formatting broken	Client strips styles	Use simpler formatting
Large file size	Too many images	Link images instead of embedding
Subject truncated	Too long	Keep under 60 characters
Emoji not showing	Old email client	Use text alternatives

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Limitations

• Email clients cannot execute JavaScript — Mermaid diagrams use table fallback
• Complex CSS layouts may render differently across clients (Outlook vs Gmail vs Apple Mail)
• Inline images add to email size — consider linking for large image sets
• Some clients strip CSS — design for graceful degradation

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Requirements

• Node.js 24+
• pandoc (for Markdown to HTML conversion)
• Shared modules: markdown-preprocessor.cjs, mermaid-pipeline.cjs

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Muscle Script

.github/muscles/md-to-eml.cjs (v1.0.0)

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Conversion Acceptance Decision Table

Condition	Verdict	Action
Subject, From, To headers present and correct	Accept	Required fields for valid .eml
Missing or malformed email headers	Reject	Check frontmatter extraction
HTML body renders in Outlook/Gmail preview	Accept	Test in at least one client
Body shows raw HTML tags or broken layout	Reject	Check MIME Content-Type is text/html
Inline images display (CID or base64)	Accept	Verify no external URL references
Images missing or show broken icons	Reject	Embed as base64 or CID attachment
Links are clickable and point to correct URLs	Accept	Spot-check 2-3 links
Mermaid diagrams pre-rendered as inline images	Accept	Raw mermaid syntax is not email-safe
Mermaid diagrams show as code blocks	Reject	Pre-render to PNG before embedding
File opens in default mail client without errors	Accept	Test .eml file import
Total .eml size >5MB	Warning	Compress images or link instead of embed
Plain-text MIME part included as fallback	Accept	Recommended for accessibility

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Related Skills

• md-to-html — Sister converter for web page output
• md-to-word — Sister converter for Word document output
• md-scaffold — Generate email templates with correct frontmatter
• lint-clean-markdown — Pre-validate markdown before conversion

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Skill version: 2.0.0 | Last updated: 2026-04-14 | Category: document-conversion