Watchguard V10 User Manual page 328

CHAPTER 10: Using Proxies
Masquerade Message IDs
Deny Message tab
This tab allows you to customize a Deny Message. The
Deny Message replaces messages that are denied.
You can customize the Deny Message with standard text.
You can also change the character set, for non-English text,
and you can call values from the proxy action to describe
why content was removed.
The following values can be called from the proxy action:
%(type)%
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Select this checkbox to replace the Message-ID
Header field in all outgoing messages. Note that
this may disrupt message threading.
This inserts the Content-Type for the content that is
stripped.
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