Changelog:
- Reject invalid custom headers that are empty or contain breaks
- Various fixes for DKIM issues, especially when using mail() transport
- Drop the l= length tag from DKIM signatures; it's a mild security risk
- Ensure CRLF is used explicitly when needed, rather than static::$LE
- Add a method for trimming header content consistently
- Some minor tweaks to resolve static analyser complaints
- Check that attachment files are readable both when adding and when sending
- Work around Outlook bug in mishandling MIME preamble
- Danish translation improvements
Sponsored by: Netzkommune GmbH
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