Thanks
reinob for your reply, which does indeed clarify it.
That's right; it's my server running on their infrastructure where my IP isn't shared with other users so it looks very much as though I'll have no alternative but to sign up to Microsoft's "Junk Mail Reporting Program."
@dotlj
Thanks for your reply too. By the way, I was amused you used Alice, Bob and Mallory, which I hadn't come across before, to illustrate the problem of changing IPs, so I read up about them
here. I hadn't come across
ProtonMail either, but I must say it looks first class.
Finally, the author of the
ISPmail_tutorials, voices his opinion in no uncertain terms:
In the previous tutorial for Debian Jessie I had a lengthy comparison of self-signed certificates, company PKIs, LetsEncrypt and paid certificates. Let’s cut it short – we will use LetsEncrypt. There is no reason to pay the certificate mafia money any more. Why do I consider them mafia-like? Because it is plain wrong to exchange money for trust. And the recent history of awkward failures shows that they deserve no trust.
Any way to save one's hard-earned money is a boon if you ask me.