So far you will have received warning on the SSL certificates you use for Postfix, Dovecot and the RoundCube email web interface. SSL/TLS is a great way to automatically encrypt the passwords between the email user and your mail server. So you want to have proper certificates. There are three ways you can handle your certificate:
Either: Leave it like it is
The users will receive a warning that the certificate is invalid and likely does not even match the name of the mail server.
If you already have a server with Debian on it and cannot or don't want to reinstall then feel free to skip this page. This may be the case if you are using a (virtual) root server from a hosting company or you do not have console access.
In a previous chapter we made sure that Postfix knows which emails it's allowed to receive. Now what to do with the email? It has to be stored to disk. Usually that's done by Postfix itself which comes with a very basic mail delivery agent (MDA) called "virtual" that just saves incoming emails to virtual mailboxes on your hard disk. But as we will use Dovecot (for IMAP and POP3 access) anyway we can use its more featureful "local delivery agent" (also known as "Dovecot LDA").
Postfix distinguishes between three kinds of domains. This is a very important concept that you need to understand. Probably half of the support request of desperate readers is caused by misunderstandings here. This page is just for learning - there is nothing to be done on your server yet. See also the documentation on virtual domains on the Postfix website.
Now it's time to prepare the MySQL database that will store information that controls your mail server. In the process you will have to enter SQL queries. You can enter them on the 'mysql' command line. But if you are less experienced with MySQL I suggest you start easy with "phpMyAdmin" by pointing your web browser at this URL: http://YOUR-MAIL-SERVER/phpmyadmin. You should see a web page like: