To prevent mail routing loops, we included the options to support normalization RFC 3834.

According to Section 5 of RFC 3834:

The purpose of the Auto-Submitted header field is to indicate that the message was originated by an automatic process, or an automatic responder, rather than by a human; and to facilitate automatic filtering of messages from signal paths for which automatically generated messages and automatic responses are not desirable.

Note: You can enable/disable this feature in the options, section 'Advanced'.