The way to do this is to create a new global variable (I’ll call it global.body) with the full contents of the body of the email. Then, perform the following steps:
If global.body starts with External Email -- This message is being sent from a user outside the organization, overwrite global.body with the contents of global.body after the text you want to remove.
If global.body contains "You don’t often get email from …, overwrite global.bodywith the contents ofglobal.body` after the text you want to remove
You can do all of this in a rule at the beginning of your mailbox (the first rule in the list of rules), and then you can use {{global.body}} within any rule in that mailbox.