Automated emails generated from activities from address
I think civicrm uses the setting from civicrm/admin/setting/smtp?reset=1, i.e, Administer -> System Setting -> Outbound Email 



If the above setting is enabled, civi uses the creator of the activity as a from address in all the email notification.

Set this to No in order to fix this problem.

Note that the above setting acts on your complete site and cannot be configured only for activities.