is there an easy way to change this behavior in a local instance?

I see. I missed that additional email addresses can be added. Thanks.

Your email to Phabricator was not processed, because an error occurred while
trying to handle it:

This email was sent from "Jane Doe <jane.doe+phabricator@gmail.com>",
but that address is not recognized by Phabricator and does not correspond to
any known user account.

Phabricator is also not configured to allow unknown external users to send
mail to the system using just an email address.

To interact with Phabricator, add this address ("Jane Doe <jane.doe@gmail.com>")
to your account.