You would have received a login URL and password to access PostfixAdmin for your domain(s). It's not the same password used for your email account.
The most useful screen in PostfixAdmin is Virtual List. This screen shows, for one domain, what alias domains, email aliases and mailboxes are already in use. You can add, edit and delete them here.
An alias domain is for the situation where you have two domain names that have the same mailboxes. It's not used much.
A mailbox is what most people expect from an email address. First, there's an address - something like name@yourcompany.com - a destination that people can send mail to. Second, a mailbox has a storage area on the server, which stores mail and can be accessed if you have the login and password: it's like a letterbox, holding mail on the server until you pick it up. There can be more features, but with a mailbox you always have those two.
The email alias is also an address, but it doesn't have the storage - instead, it will forward mail to another address.
From the Virtual List screen, you can:
You'll see Quota mentioned throughout Postfixadmin. Quota is a method of limiting the amount of storage that a mailbox can use, but it doesn't do anything on this system.
Passwords: When setting a password, please take care to write down the new password, or store it somewhere. You can change passwords in PostfixAdmin, but you cannot find out an existing password. This is a deliberate security feature. Best practice is to store passwords in a purpose-made password manager app on your PC, such as KeePass.
If you want to allow your users to change their passwords, there's a second control panel they can use. They would login at https://mail.rivwe.com/pfadm/users/ using their email address and email password. This also allows users to change their email forwarding and set up an autoresponse: for their own address only.
PostfixAdmin requires passwords to be a mix of letters (upper and lower case) and at least two digits 0-9. It's not good practice to let account owners choose their own passwords: this isn't a password they have to remember because it will be stored by their email app, so it doesn't have to be easy to remember. These passwords do have to be safe ones.
If you need to test whether a password works, use it to login on the webmail. The webmail doesn't have any special access to the mailserver, it uses IMAP, just like Outlook etc.
Creating a new account: it's easy to leave the password fields blank. PostfixAdmin will generate a password and display it: be sure to record it in your password manager app. Postfixadmin will create the address and mailbox and send it a simple test message. You can give the new address/password to its new owner, and/or set up their email app. Here are the email account settings for this server.