One approach could be to create an Automation Rule that is triggered on Save of the Message that is created to represent the email:

Code:

# custom_ray_odoo

discussions_subtype = env['mail.message.subtype'].search([('name','=','Discussions')])

for record in records:
  if record.model and record.res_id and record.subtype_id == discussions_subtype \
                  and "Also sent to:" not in record.body:
    document = env[record.model].search([('id','=',record.res_id)])
    followers = document.message_partner_ids
    header_note = ''
    for follower in followers:
        if follower != record.author_id:
          header_note += follower.name + ' (' + follower.email + '); '    
    if followers and document:
      record['body'] = 'Also sent to: ' + header_note[:-1] + '\n\n' + record.body