The Civimail Activity Type is Send Email. This is somewhat confusing with Email Sent for emails sent from the contact record form. Either all sent emails should have the same activity type (maybe better for searching for all email correspondence) and the sub-type will be explicit in the link; or call the CiviMail Activity Type "CiviMail Email" for clarity. If it's done the latter way, keep the word Email in the type, so that all emails can be searched for using a wildcard %Email% search. Then also any new module that for example tracks incoming emails might be of type Email Received.