I think a UI that makes it easier to pick a server (by domain name) would be the easiest way to do this. Something like:
Profile name
(profile image, autogenerated but replacable)
Server 
(combo box, choose from dropdown list or type-in, or scan server-invite QR)
This is a Chatmail server, an account will be automatically generated for you.
Server policy is: autoloaded policy
[Agree and Create profile]
OR
If it is not a Chatmail server, offer options as on current “Log in with an existing e-mail account” screen.
(asks e-mail addy and password, offers show-classic-mails option and advanced manual server config) Add tickbox forcamoflaged Chatmail servers.
The publically-listed Chatmail servers announce to the world that they are Chatmail servers, so if the user chooses a server from the drop-down list, the UI can say “This is a Chat mail server…” and the server-specific policy can be auto-filled. Additional server info, like the host country (flag) and domain-name registrar, could also be provided.
If the server does not announce that it is a Chatmail server, there should be a tickbox to say it is nonetheless a (camoflaged) server.
The current setup has a New/Existing choice, then a Create/Other choice, and “Other” gives four more choices. This would give one page with two choices, profile name and server name, asking more info only if and as needed.
Sorry for the terrible mockup, I can draw better if there is any interest.