The fastest and easiest solution is to create a Flow for the Email Message record that strips text from email messages. If desired, include the Case Comments object as well.

Steps to create the Flow:

• • Go to Setup > Flow > New Flow.
  • Select Record-Triggered Flow.
  • Click Create.

• Configure the Trigger

  • Object: EmailMessage.
  • Trigger: When a record is created or updated. Note: Salesforce doesn't allow the email body field to be updated after insertion, so it's most likely that it will only ever trigger on creation.
  • Set Entry Conditions: Specify conditions (if any) to target the email messages you want to update.
  • Optimize for: Fast Field Updates if the change is only to the email message fields, or Actions and Related Records if other records need updates too.

 

• Add a Formula Resource

  • Go to the Manager tab and create a Formula Resource.
  • Data Type: Text.
  • Formula: Use SUBSTITUTE() to remove or replace the unwanted text. For example, use the following formula to replace "unwanted text" in the email body with an empty string:
    
    • SUBSTITUTE({!$Record.TextBody}, "[Image no longer available]", "") 

 

• Update Records

  • Drag the Update Records element onto the canvas.
  • Select Specify conditions to identify records, and set fields individually.
  • Object: EmailMessage.
  • Field to Update:
    • Field: TextBody or HTMLBody (depending on where the text appears).
    • Value: The Formula Resource you created.

 

• Save and Activate the Flow

  • Save the Flow with a descriptive name (e.g., "Remove Text from Email Messages").
  • Click Activate.