pull nixos-mailserver module into this repo

nixos-mailserver/README.md

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+Mailserver
+----------
+
+This is a generic mailserver module for NixOS.
+The following components are used:
+- Dovecot (IMAP, ManageSieve, LDA)
+- Postfix (MTA, MSA)
+- Postfix Admin (User administration through a web interface)
+- Nginx (Webserver for the web components)
+- Roundcube (Webmail)
+- Rspamd (Spam filter, DKIM, etc.)
+
+
+Initial Setup
+-------------
+Configure the options in services.mymailserver as needed:
+You must configure DNS as follows:
+The values of pfaFQDN, mailFQDN and roundcubeFQDN must point to
+the mailserver. pfaFQDN and roundcubeFQDN can be CNAMEs pointing
+to mailFQDN to ease administration.
+The server will automatically try to get HTTPS certificates for
+these names through ACME / Let's Encrypt.
+
+To configure your new mailserver, substitute pfaFQDN in the following
+URL: https://pfaFQDN/setup.php
+Enter the setup password that you want to use in the form and 
+click on Generate setup_password hash.
+Next update the configuration.nix for the mailserver and set
+pfaSetupPWHash to the hash shown by the website, for example:
+pfaSetupPWHash = "$2y$10$EdLjCOTehuPtIsfZRytTYeH20pW/LA73baFPhY05nHkGk2XpPv3Zu";
+Rebuild and switch the mailserver config, then access the same url
+again and enter your password. Scroll to the bottom and configure
+your super admin account.
+Once that is done, it is a good practice to remove the pfaSetupPWHash
+from the configuration to disable the setup.php. Remember to
+rebuild and switch your NixOS configuration again.
+Now log into Postfix Admin with your new superadmin account:
+https://pfaFQDN/
+Setup admin accounts, domains and mailboxes as required.
+Make sure to create the mailbox for the adminAddress if it's
+a local mailbox. RFC requires that address to be monitored.
+If you ever change the adminAddress, make sure to update
+all aliasses as necessary, as adminAddress only affects newly
+created domains in PostfixAdmin.
+
+DKIM keys need to be generated manually through the dkim-generate
+script.