Eventually and in complete despair, I deleted all local and remote files (except the 9 you have rewritten) and installed PM from scratch. I have done that twice.
[I. A] the use log in screen insists on reporting an invalid password.
[I. B] if I click the "Register" button on that log-in screen, there is an immediate 500 error.
[II. A] The backend works fine; no roblems with logging in creating mailings, etc.
[II. B] But unfortunately, the whole is back to refusing to send emails.
Thank you, and I am again sorry for the toruble.