I might be wrong, but for me OP is not trying to actually run email, rather have a staging ground that pulls all emails from their accounts on actual providers, apply some treatment to said emails and pull them from client-side apps on their devices.

I haven’t tried any of the following, just my 0.02.

Idea0: is there any way to install your instance without using the setup wizard ?

Idea 1: installing and older version (below 6.x) and upgrading it to current version ?

The official documentation page about registration states:

When deploying a self managed Rocket.chat >=6.x workspace you are automatically required to register your workspace upon completing the Setup Wizard

According to the same page, for versions below 6.x you have to manually register your instance.

Idea 2: I noticed a few references to air-gapped installations, and I guess they can’t be connected to anything from Rocket.chat HQ. Could Iistalling your server as air-gapped and completing a bogus registration work ?

Official documentation page about Air-gapped installs registration

Hope any of this helps. I’m quire surprised (and not in a good way) to see this mandatory requirement from Rocket.chat. Even if kt means functionality loss (access to their push notif framework for example) you should be able to decline it.