Many mail clients have the option of marking an outgoing message 'high priority' or 'urgent.' In Mail.app, such messages show up in the Inbox with two exclamation marks (!!) in the flags column. If you want to sort messages by the flags column, so that messages you flag yourself are at the top, all high priority messages you have ever received will also be at the top of the inbox.
Mail.app currently has no way of removing this flag from emails you have received (as noted in this Apple discussions thread.). The following shell script allows you to unset the priority status of all the emails you have ever received, so that no emails in your inbox have '!!' next to them any more:
sqlite3 ~/Library/Mail/'Envelope Index' 'update messages set flags = flags | 196608 where flags & 196608 != 196608;'
do shell script "sqlite3 ~/Library/Mail/'Envelope Index' 'update messages set flags = flags | 196608 where flags & 196608 != 196608;'"
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