I want to start an email service provider like gmail, outlook, protonmail

[joy223]

I want to start a paid email service provider. I have about 5 customers already who will sign up. I plan to charge 20 dollars a year for 1 email and 50GB of space.

I bought a domain. How can i get started?

will roundcube allow anyone who goes to my domain and register up. My plan is just to put on the domain homepage a link to paypal so they can pay 20USD then when i receive payment i will manually approve their email account.

I installed roundcube and i already see the login page. How do i access the admin panel so i can open up registration?

[Dmitry42]

Roundcube its just client like Outlook, Thunderbird.

So users and services part of your mail server you must create and setup independently form RC

[joy223]

I don't understand. Do you mean I can't create users in the admin panel? I want to create some users and give them the password.
They can just login through my domain that I setup.
The domain will just be roundcube.
They plan to pay me with cash because they live near me.

[JohnDoh]

Do you mean I can't create users in the admin panel?

Roundcube does not have an admin panel. Roundcube is an web based IMAP client. How you setup and manage your users is out of scope for this forum.

[VA1DER]

If you don't already know what's involved in this, then it's likely outside your experience level to accomplish.

What you will need is to configure a server for a virtual domain email provider using something like postfix, dovecot, mariadb, phpmyadmin, and postfixadmin (this is the user-creation control panel you were speaking of) and then roundcube.  This is very complicated and very easy to get wrong in ways that leave your system vulnerable.  It is also difficult to get the details right - you need to think about milters for DKIM, SPF, and spam filtering.  You will need to have a TLS certificate mechanism, which likely means engaging letsencrypt and using Acme.  You need to have a good knowledge of domain infrastructure even if you don't run your own DNS server, because you need MX records, TXT records for the DKIM and SPF, etc.  You will need it to run on a system with a static IP address that your provider will create a reverse-DNS entry for.  Without a reverse DNS entry that points back to your own domain, you can kiss goodbye any hope of sending mail to any major player that doesn't get trapped in their own spam filters.  And even after all this, you will need to operate for a year or two so that your domain develops a mail reputation significant enough for email that your users send out to not end up in Google's spam filters.  Google is not kind to new email providers, and email your users send to gmail users will, even if you do everything right, end up in those users junk folders for at least a year or two.

You can try for a turnkey solution like mail-in-a-box, but even this can be less than ideal and you still need to understand all of the above.

Roundcube is a very small part of running an email service.

Just as an aside, the numbers you are quoting are likely going to be expensive.  No VPS provider you can use will give you enough space you can just hand out 50GB-per-user to.  And this just isn't something you can do running off a computer hooked up to your home fibre.  You will never get a reverse dns PTR record for any home internet IP address.  You will need to rent server colocation, which you'll be looking at ~$200/mo.  So you'll need about 120 users to break even.  And that's about 6TB of storage right there, so factor that into your hardware costs.