Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.
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the transport rule injects the word [EXTERNAL] into the subject line.
If a user repsonses, Outlook repeates the actual subject line an puts its own in front of it (default).
So you have some options in Outlook (Tools - Options - Email) how Outlook interacts with this.
As far as not set via policy, I guess you do not have really influence on this setting.
The only chance I would see is to create another transport rule to filter is out again.