It is important to keep in mind not all email systems implement auto responders.
To be more specific, that is a feature still in draft mode, governed by two Internet Engineering Task Force (IETF) standards:
RFC-3834 Recommendations for Automatic Responses to Electronic Mailand
RFC-5230 Sieve Email Filtering: Vacation ExtensionThat may not be enabled or configured on systems implementing auto responders.
It is up to each email server administrator.
In general most systems will send the auto reply to the intended "reply to" address.
Some will send to "from" regardless what is set on "reply to".