It looks like you're trying to take an HTML formatted message, strip the HTML out, and then send the remaining text out without two pieces.
Here's a flow which accomplishes this:
1) The HTML To Text action takes the Email Body
2) The Compose runs the expression from
@chiaraalina but targeting the HTML TO Text action instead of the original email body:
replace(
replace(
body('Html_to_text'),
'CAUTION! This email originated from outside of our organization. DO NOT CLICK links or open attachments unless you recognize the sender and know the content is safe.',
''
),
'This email contains an automatically generated notification created by your organization.',
''
)
3) I don't have access to the same Send A Message action you have, but you'll reference the dynamic value Outputs from the previous Compose action as the message, since that's what removed the text
This should accomplish your goal.
If this comment resolves the issue, please remember to mark it as the answer.