Here is a clue on those webmail client gui’s that are easy to tweek and those that are not - perhaps Roundcube is too much of a heavy weight to be customizable:
"Modoboa provides a simple webmail"
https://modoboa.readthedocs.io/en/1.2.0/plugins/webmail.html
Servercow is their managed mailcow service. Mailcow.email is their website for Mailcow.
In a nutshell … a managed MailCow instance for 40 EUR a month with 100gb storage and up to 35 active sync devices. Seemingly no limit on domains or on email aliases and users. Free daily backups.
But you are not interested in them hosting it for you … so why did I translate ? smh
I don’t see any e-forum and don’t even see an email there, so I visited their commercial e-site Servercow.de and there was an e-mail address there.
Mailcow also like Miab uses Roundcube https://mailcow.github.io/mailcow-dockerized-docs/third_party-roundcube/ and this Roundcube is a heavy weight software to tweek, this guy did it in 2006 and it was not easy https://www.roundcubeforum.net/index.php?topic=135.0
https://modoboa.org support plans
Ouch! Price €800.00 €1,200.00 €3,200.00 €7,500.00
They do their support on IRC.
Looks to me like they use SoGo as the webmail, not Roundcube.
Never been on Irc because heard it is not very secure.
Sorry I could not find SoGo on their e-site, instead saw RoundCube.
It is in their documentation:
Anyways, enough talk about MailCow! 
I don’t know this meaning on their tagline below the logo, do you know?
" The mailserver suite with the ‘moo’ – 
+ 
= 
"
Subject: Re: Mailcow & Servercow
To: andre.peters@servercow.de
Please tell me the meaning about this:
mailserver suite with the ‘moo’ –
Don’t understand the whale meaning? The dash / hyphen after the ‘moo’ looks as if it’s a subtraction or minus sign. Confusing!
I suspect it represents the docker logo. But as Alento said, this is not a forum to talk about mailcow.
‘moo’ – 
+ 
= 
Means “mailcow plus docker equals awesome.”
Alright so mailcow aside, I still think MIAB will not work for you in the long run, if mailcow or other easy installers do not work maybe try manual installation?
Toner,
Evaluating all the messages, my approach should be:
write a plan with your idea and the required scale.
find some budget for creating the final plan (running a “freemail” service is really more then just installing mail server and some scripts. Website, marketing, writing the right conditions, “user support”, even if it’s only online support, it will require human intervention. If successful, you need the power to grow to the desired scale, probably in a very short period of time.
rent a VPS with enough IO handles for the amount of users; there is a difference between handling up to 100 users or handling 100k+ users, although in basic it’s the same; for less then 100 users, a small VPS for the start
install MiaB
install the script from Murgero (with some small extensions for registering additional information)
and try out.
next, use a second vps to test different mail clients.
and if your idea is final, just start.But you will have to compete with the “googles” of this earth, which means that users expect some continuity of the available of their email (and response).
finally, you will need a manual installation, based on growth, but also on your other plans. For example, do you want to provide other services to end-customers.
based on your plans, you have to decide your first steps for testing up to - for example - 1000 … 1500 users.
MiaB is perfect as a complete (standalone) mail platform.
I’m configuring and maintaining mail servers since 1994 (started with sendmail), but the last 6 years, my focus is more on software development, so about 4 years ago, i installed MiaB.
As some others on this page, I also can install custom installations, but it’s also a living for me;) If you need some support, you can also contact me with a pm.
Regards,
Sander
I should print this out, frame it and hang it on the wall. Thanks a lot for this.
I really hope, I put you in the right direction and didn’t write down your idea to the trashcan.
people who offer free mail, such as cock.li truely deserve much gratitude!
They are up against a world of enemies:
requirements for surveillance can cost you much time and money. so better base the business on the Seychelles or somewhere proper.plus some weirdos will run attacks against freemail as if it were M$ or whatever.
Also you hear pro’s complaining how Big Data simply trashes or mark spam perfectly legit email, just because it comes from a small provider, breaking all RFC rules, to discourage customers from fleeing Google-mail like hostages. 
Sadly, it is all true. 
What ever happened to net neutrality?
Oh wait, small providers aren’t contributing to their bottom line. Ugh.