After several hours banging my head against the wall over an Outlook error “0x800CCC80; None of the authentication methods supported by this client are supported by your server” I finally found the cause of the problem; I had installed the postfix mail server on the new Plesk VPS host rather than qmail.
Postfix requires TLS encryption on port 587, qmail does not – and the Outlook default is “None”. Changing the encryption required in Outlook is disruptive – lots of clients – and also gives rise to a certificate warning, so I decided to take the cowards way out and replace postfix with qmail.
More here.