As discussed in https://talk.plesk.com/threads/send-email-with-credentials-to-my-customer.369632/#post-927240 Plesk should have a button for server admins by which an admin can send a welcome mail to a user with the user's login credentials. That should be template-based with an editable template to cater for the requirements of different server providers.

It should also not be sent directly, but similar to the existing "mass mailing" function first show the proposed text in an editor window so that the admin can make additions or edit the text. Then send from there.