Ostensibly the facility appears to be connected with a strong “civil liberties” social position but, looking over the 13+ years we have been going, I see about 30 cases and all of them were dodgy in some way 
We already do not allow the use of disposable email domains (about 2000 of them).
protonmail.com and proton.me are not dodgy or throwaway in any way. They are domains for the Proton email service, a valid and popular free and payable email service just as is gmail.com.
Proton is known for their security and privacy, with several popular services in addition to email.
What they do have as part of their service is an alias capability which allows control over various advertising web sites from which it is almost impossible to unsubscribe once an email address is provided. The most common domain used by Proton for this is passmail.net (see „hide-my-email alias section of the first link above. I understand your issue with this sort of alias and it is fair to insist that Euroga.org users not use this sort of address.
But don‘t consider protonmail.com and proton.me to be in that category.
Surely every email service, whether it is a pop or imap mailbox, or webmail like gmail, is private?
All of them will reveal all on receipt of a court order, unless hosted in Russia.
And if someone wants court order protection then you can drawn your own conclusions 
There are lots of reasons these days to protect oneself from hackers, trackers, and others with malice intent or for using your information to generate revenue… read: Google. Not everyone who wants to keep their data private and secure is a criminal.
Further reading:
Privacy
Transparency
I understand your issue with this sort of alias and it is fair to insist that Euroga.org users not use this sort of address.
It isn’t “my” issue; it is an issue for all EuroGA users, because if someone signs up with an email address which later becomes dud, and subscribes to various threads, then the notifications bounce, and over time EuroGA gets classified as a spammer, and load of other users’ notifications don’t get delivered by their ISPs.
I see passmail.net used by only 2 people in 13 years.
Peter wrote:
Why use protonmail.com or proton.me email address?
It’s for people who don’t want others to read their email.
Surely every email service, whether it is a pop or imap mailbox, or webmail like gmail, is private?
All of them will reveal all on receipt of a court order, unless hosted in Russia.
AFAIR a Proton mailbox is client encrypted. Host is unable to read mailbox content. Of course mail needs to be sent and received, so one trusts that Proton discards the data after transmission just like VPN operators claim they do.
Peter wrote:
It isn’t “my” issue; it is an issue for all EuroGA users, because if someone signs up with an email address which later becomes dud, and subscribes to various threads, then the notifications bounce, and over time EuroGA gets classified as a spammer, and load of other users’ notifications don’t get delivered by their ISPs.
There is no reason to expect this to be more common with Protonmail than with gmail.
Airborne_Again wrote:
There is no reason to expect this to be more common with Protonmail than with gmail.
Was going to write something similar. Probably the accounts expire after a period of inactivity and thus “go dud” if the user stops using them. Or the user deliberately deletes his account. Something which is quite an effort with the other popular services like gmail or hotmail where zombie accounts often live forever.
The reality is that it is “infinitely” more common with protonmail – as per my post #1
Proton mail, the email provider of choice for a plethora of criminals in my experience. Having seen proton mail present at a law enforcement conference, I think most users would be surprised about the amount of assistance they provide on request.
Tools like proton mail present an image of being impervious but have a fatal design flaw that they try to hide. They might not have your email key but it’s trivial for them to get it by changing their site code. WhatsApp is currently facing a similar accusations about their end to end encryption.
