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New articles: - Python smtplib: Missing Date/Message-ID Headers Break Mail Clients - Fantastical MCP: Permission Denied (macOS Quarantine) - Ubuntu dist-upgrade Repo Quarantine Updated: troubleshooting index, SUMMARY.md nav, WOL article edits. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
60 lines
2.3 KiB
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60 lines
2.3 KiB
Markdown
---
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title: "Python smtplib: Missing Date/Message-ID Headers Break Mail Clients"
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domain: troubleshooting
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category: general
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tags: [email, python, smtplib, spam, rfc, spark]
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status: published
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created: 2026-04-29
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updated: 2026-04-29
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---
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# Python smtplib: Missing Date/Message-ID Headers Break Mail Clients
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## Problem
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Emails sent via Python's `smtplib` and `EmailMessage` appear on some mail clients but not others. The emails are delivered to the server and visible in Maildir, but specific clients silently suppress them.
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## Root Cause
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Python's `EmailMessage` does **not** automatically add `Date:` or `Message-ID:` headers. These are required by RFC 5322. Without them:
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- **SpamAssassin** flags `MISSING_DATE` and `MISSING_MID`, and may set `X-Spam-Flag: YES` even if the overall score is below the spam threshold
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- **Mail clients** (e.g., Spark) may filter on the spam flag header and silently hide the message — no Junk folder, just invisible
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- **Other clients** (e.g., iPhone Mail, some Spark builds) may be more lenient and display the message anyway
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This creates a confusing situation where the same email appears on one device but not another, despite both using the same IMAP account.
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## Fix
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Always include `Date` and `Message-ID` headers when constructing emails with `EmailMessage`:
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```python
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import smtplib
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from email.message import EmailMessage
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from email.utils import formatdate, make_msgid
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msg = EmailMessage()
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msg['Subject'] = 'Your subject here'
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msg['From'] = 'sender@example.com'
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msg['To'] = 'recipient@example.com'
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msg['Date'] = formatdate(localtime=True)
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msg['Message-ID'] = make_msgid(domain='example.com')
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msg.set_content('Email body here')
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with smtplib.SMTP('mail.example.com', 25) as s:
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s.send_message(msg)
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```
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## Verification
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After applying the fix, check that SpamAssassin no longer flags the headers:
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```bash
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# Check email headers on the mail server
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grep -E 'MISSING_DATE|MISSING_MID|X-Spam' /var/vmail/domain/user/cur/<message-file>
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```
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A clean message should show `X-Spam-Status: No` with no `MISSING_DATE` or `MISSING_MID` in the test list.
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## Key Takeaway
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Python's `EmailMessage` is a low-level builder — it trusts you to set all required headers. Unlike higher-level mail libraries or webmail interfaces, it will happily send a message with no date or message ID. Always add both explicitly in any script that sends email via `smtplib`.
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