Error "This email template contains too much text" on Salesforce List Emails

When we copy text from external sources (for example, MS Word or MS Outlook) and paste it directly into the Salesforce email composer, the formatting from the external source is also being carried over. Typically, this formatting can reach a point where it hits the HTML Body character limit when sending List Emails (32,000 characters).

When emails are too long and exceed the HTML Body limit, the following error message can appear:

"This email template contains too much text. Remove some text and try sending again. Or if you decide to send it anyway, some text will be cut off in the received email."