What the use case apart from commercial spam here? If you even have one, elucidate it clearly and make this service impossible to use for sending spam emails impersonating humans.

As one example do what you could do to prevent spam, humans should have to opt in to receive email from this service. If it is useful they will and this is in fact required by law in many jurisdictions.

Otherwise your servers will be blacklisted for illegally sending spam and you will deserve it.