[Bug 490935]

[Bugzilla-kabsi]

Apple Mail and MailTags can do this.

MailTags provides keyword tagging and adding of notes to mails. Notes
can be set as subject. Full text search / Spotlight will find text in
notes, too.

See http://www.indev.ca/MailTags.html

-- 
it is not possible to modify email subject
https://bugs.launchpad.net/bugs/490935
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