I understand the mail sending mechanism is currently realised through the mediawiki core, but it would be great to be able to send forms to an arbitrary email address rather than a MediaWiki user account.
Among other possible use-cases, that could help mitigate the leakage of information about the persons whose wiki accounts are assigned to the recipient of the form, when it is undesirable to know who the person is.
Currently, when a submission is made using the form with the submitter requesting a copy of the form submission, the subject of email sent to the submitter is the content of the gettext string identified as "emailccsubject", i.e. "Copy of your message to $1: $2", which reveals the username of the recipient.