move espect into Postal::MessageInspection

app/models/incoming_message_prototype.rb

@@ -87,7 +87,7 @@ class IncomingMessagePrototype
       mail.to = @to
       mail.from = @from
       mail.subject = @subject
-      mail.body = @plain_body
+      mail.text_part = @plain_body
       mail.message_id = "<#{SecureRandom.uuid}@#{Postal.config.dns.return_path}>"
       attachments.each do |attachment|
         mail.attachments[attachment[:name]] = {