When sending emails from the Activity History related list in the Case object, emails are limited to 384,000 characters. When emailing from the Case Feed or from the Case Publisher, the default limit is 131,072 character.
To learn more about the differences in email character limits, please review Number of Characters allowed when composing an email using the Standard Email body field or Email Publisher in the Feed.
If the email content is typed manually in the feed, Salesforce will not allow Users to type any further if the 131,072 character limit is reached. However, if Users are copying and pasting email content from other resources (for example, Microsoft Word), full email content will be copied to the email body, but Users will get the following error message when they click Send:
"Error: You cannot use more than 131,072 characters"
You have two options on how to avoid or remove the error message:
000386909

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