email to case auto-responder and contact emails
Email to case matches on all email fields, including custom email fields.
However, if email to case finds a matching contact, the auto-response email appears to always include the contact’s preferred email, which is problematic if one thinks about it for a few nanoseconds. Contact submits a case from email abc and the autoresponse is sent to email xyz (and also email abc, if you check the box to copy all email addresses on the autoresponse).
There is now an easy declarative way to remove the contact from the case, using before-save flows, which fires before the auto-response rule!
The flow needs to set case.contactId and case.preferredEmail to blank (global constant empty string). I assume both are required, did not test them independently.