Email fails domain authentication

Error message seen, sometimes in conjunction with an email going in to junk email. 
'This email has failed its domain's authentication requirements. It may be spoofed or improperly forwarded!'

How you would notice
You've drafted your email and send a test to your own email (protonmail.com is particularly sensitive to this error), and you have this error message shown above. Your test email may have also gone into your Junk/Spam folder
Reason
Something wrong with the reply-to email. In particular proton.me email addresses are not recommended
Temporary fix
Try sending another test with noreply@extinctionrebellion.uk in the 'Reply-to Email' field. If this works - then that is a sign that the system (or the internet!) doesn't like your reply-to email address.
Permanant fix
Use a different reply-to email address. If you are part of an XRUK central team, campaign group or local group you can apply for an xxxx@extinctionrebellion.uk email address which should work much better for you as we have registered 'extinctionrebellion.uk' with Action Network as a valid 'domain' to stop this kind of problem.