Normally the ISP supports TLS and credentials need to be used. Any idea why this is not working? As you may understand, I prefer encrypted e-mail.
TLS means Encrypted email, so there’s no problem.
Credentials only need to be used when a client (the email owner, through Outlook etc.) connects to the mail server to download email from a mailbox, or when it wants to send an email outside.
When a mail transfer agent (the postman) connects to a mail server to deliver a mail to a mailbox, it doesn’t need to enter credentials (unless special protected mail servers ask to do so).