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Sorry - I re-read your initial today and realize that (perhaps?) you only want to encrypt forwarded mail if the original is already encrypted.
If it's only the forwarded emails that you want to encrypt, you'll want to look at the current mailfile (template) to see the differences between the 'Forward' action, and the 'Reply' action.
First glance it appears that the 'Reply' opens a reply form, the Reply form has code that checks to see if the original was encrypted (field 'Encrypt' = '1'). The forward action appears to open the memo form, and doesn't appear to include the same checks.
Feedback response number WEBBAUGQQ4 created by Mark Taylor on 12/28/2017