Ruby script to convert a mail (.eml/.emlx) file to a note in macOS Notes.app.

mail_to_notes.rb

require 'base64'
require 'digest'
require 'mail'
require 'nokogiri'
require 'shellwords'

file = ARGV.first
puts "Processing file #{file}"
contents = File.read(file)

mail = Mail.read(file)

subject = mail.subject
date = mail.date.to_time.utc
from = mail.from.first

folder = (subject.match(/#([\w ]*)/).captures.first.chomp rescue "Notes").capitalize

searchID = date.strftime("%s")
subject = subject.sub(/\s*#[\w ]*$/, "")

puts "Creating new Note with title \"#{subject}\" in folder \"#{folder}\""
puts "Received on #{date} from #{from}"

match = contents.scan(/filename\s*=\s*([^\n]*)\n[\s\S]*Content-Id:\s*<([^>]+)>/mi)

attachments = Dir.chdir(File.dirname(File.dirname(file))) do
	attachment_globs = match.map { |f, _id| "**/#{f}" }
	hashes = Dir[*attachment_globs].map { |found| 
		{ match.detect { |f, _| found.end_with? f }[1].downcase => File.join(Dir.pwd, found)} 
	}
	hashes.reduce({}, &:merge)
end if match

attachments ||= {}

html = Nokogiri::HTML((mail.html_part || mail.text_part || mail.body).decoded) { |c| c.strict }

abort "Couldn't find a valid html body" unless html

body = html.css('body').first
abort "The body doesn't actually contain any body tags, aborting..." unless body.to_s.include? "<body"

linked_attachment_count = 0
attachment_nodes = body.css('img')
attachment_nodes.each do |a|
	next unless a['src'].start_with? "cid"
	content_id = a['src'].downcase.sub(/^cid:/, "")
	path = attachments[content_id]
	mime_type = `file --brief --mime-type #{path.shellescape}`.chomp
	a.add_next_sibling "<en-media hash=\"#{Digest::MD5.hexdigest(File.read(path))}\" type=\"#{mime_type}\" />"
	a.remove
	linked_attachment_count += 1
end

abort "There were more attachments in the source than in the actual HTML (#{attachments.length} > #{linked_attachment_count})" if attachments.length > linked_attachment_count

note_xml = Nokogiri::XML::Builder.new do |xml|
	xml.send(:"en-note") {
		xml.div {
			xml.p 
		}
		xml.font(searchID, color: "#FFFFFF", size: "1", style: "font-size:1px;")
	}
end.doc

note_xml.css('p').first.add_child body.inner_html

inner_xml = <<~XML
	<?xml version="1.0" encoding="UTF-8" standalone="no"?>
	<!DOCTYPE en-note SYSTEM "http://xml.evernote.com/pub/enml2.dtd">
	#{note_xml.root.to_xml}
XML

abort "Couldn't create the XML from the provided string" unless inner_xml

outer_xml = Nokogiri::XML(<<~XML, nil, "UTF-8")
	 <!DOCTYPE en-export SYSTEM "http://xml.evernote.com/pub/evernote-export3.dtd">
	 <en-export export-date="#{date.strftime("%Y%m%dT%H%M%SZ")}" application="RubyExporter" version="1">
	 </en-export>
XML

Nokogiri::XML::Builder.with(outer_xml.at('en-export')) do |xml|
	xml.note {
		xml.title subject
		xml.content { xml.cdata inner_xml  }
		xml.created date.strftime("%Y%m%dT%H%M%SZ")
		xml.updated date.strftime("%Y%m%dT%H%M%SZ")
		xml.send(:"note-attributes") {
			xml.author from
			xml.source "Mail"
			xml.send(:"reminder-order", 0)
		}

		attachments.each do |_id, path|
			xml.resource {
				xml.data(Base64.encode64(File.read(path)), encoding: "base64") 
				xml.mime `file --brief --mime-type #{path.shellescape}`.chomp
				xml.send(:"resource-attributes") {
					xml.send(:"file-name", File.basename(path))
					xml.timestamp "19700101T000000Z"
				}
			}
		end
	}
end

result = outer_xml.to_xml

abort "Couldn't export the generated XML: #{outer_xml}" unless result

result_file = File.expand_path File.join("~", "export.enex")
File.write(result_file, result) 

import_script = <<EOF
set pathToImportItem to "#{result_file}"
set sortIntoFolder to "#{folder}"
set searchID to "#{searchID}"
set mailFilePath to "Macintosh HD#{file.tr("/", ":")}"

tell application "System Events"
	tell application "Notes" to activate
	click menu item "Import to Notes…" of (process "Notes")'s (menu bar 1)'s (menu "File")
	
	delay 1.5
	keystroke "g" using {shift down, command down}
	delay 1.5
	keystroke pathToImportItem
	delay 1
	keystroke return
	delay 1
	keystroke return
	delay 1
	keystroke return
	delay 1
end tell

tell application "Notes"
	set newNote to the first note whose body contains searchID
	set myFolder to first folder whose name = sortIntoFolder
	
	if not (exists folder sortIntoFolder) then
		make new folder with properties {name:sortIntoFolder}
	end if
	
	move newNote to myFolder
	delay 0.5
	delete (every folder whose name starts with "Imported Notes")
end tell

tell application "Finder"
	set comment of (mailFilePath as alias) to "converted to note"
end tell
EOF
import_script.gsub!(/^\s+/, "") # Remove whitespace
script_lines = import_script.lines.delete_if { |l| l.empty? }
puts `osascript #{script_lines.map { |l| "-e #{l.shellescape}" }.join " "} 2>&1`

Note: needs the nokogiri and mail gem to work.

This works by taking the mail, converting it to HTML and then saving it as an Evernote .enex file. Notes.app can import such files natively, so all it has to do with that is use an AppleScript to actually run the import.

Unfortunately, importing is not supported in the regular Notes.app AppleScript suite, so this script actually has to click through the relevant menu items. Make sure Hazel or whatever you're using to run this actually has permissions to control your Mac (in the Privacy > Accessibility system preferences).

The subject of the mail is used as the title of the note, and it even supports moving the note to a specified folder, e.g. the subject "My new note #Misc" will be created in the "Misc" folder with the name "My new note".

Update: I improved the reliability by doing the following:

1. After the note was successfully exported it adds a comment to the .eml file in Finder. This is useful if the rule was executed while the mac was sleeping
2. The note body receives a timestamp at the end (with a white font colour), and the note will be found through that. This is much better than trying to get the note by title, since Notes sometimes changes the title from what was provided, and has trouble getting the correct note when there are multiple notes with the same title.

Here's a screenshot of the Hazel rule: