Hi partner,
Sending an email is an email activity that cannot be customized or modified.
Could you provide more details?
- Do you want to see all the past conversations about a single contact?
- Or do you want to automatically add all past conversations into the body of the email when you write a new email?
In the first case, all relevant emails in the past are automatically displayed in the activity subgrid of the contact form. Click the filter icon to display only the emails.

In the second case, if the email is sent by clicking "REPLY", the content of the reply email will be added to the end of the text. Apart from this, there are no other related OOB features.

Hope this helps.
Best Regards,
Lu Hao