You could always go the barebones route and generate a mailto: link which contains all the info in the email that you usually send (in text mode of course) but

You could always go the barebones route and generate a mailto: link which contains all the info in the email that you usually send (in text mode of course) but which the admin can send from their personal account with their personal email client. I am using a similar implementation in another fediverse project. Another example is/was AWS manual account creation.