https://multiformeingegno.it/riflessioni/setup-postfix-and-dovecot-storing-virtual-users-in-mysql-database/

Install Postfix, Dovecot, postfix-mysql and dovecot-mysql from Ubuntu repositories.
	apt-get install postfix postfix-mysql dovecot-imapd dovecot-mysql

Let’s start with Postfix config. Edit /etc/postfix/main.cf:
	### Postfix SMTP Server
	myhostname = FQDN.TLD
	mydestination = localhost
	smtpd_banner = $myhostname ESMTP $mail_name
	alias_maps = hash:/etc/aliases
	alias_database = hash:/etc/aliases
	biff = no
	append_dot_mydomain = no
	readme_directory = no
	relayhost =
	inet_interfaces = all
	mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
	mailbox_size_limit = 100000000
	recipient_delimiter = +
	inet_protocols = all
	delay_warning_time = 4h

	### Virtual users/domains
	relay_domains = 
	local_transport = virtual
	virtual_alias_maps = proxy:mysql:/etc/postfix/virtual_alias_maps.cf
	virtual_mailbox_domains = proxy:mysql:/etc/postfix/virtual_mailbox_domains.cf
	virtual_mailbox_maps = proxy:mysql:/etc/postfix/virtual_mailbox_maps.cf
	virtual_mailbox_base = /var/www
	virtual_mailbox_limit = 512000000
	virtual_minimum_uid = 5000
	virtual_transport = virtual
	virtual_uid_maps = static:5000
	virtual_gid_maps = static:5000
	local_recipient_maps = $virtual_mailbox_maps
	transport_maps = hash:/etc/postfix/transport

	### Authentication
	smtpd_sasl_auth_enable = yes
	smtpd_sasl_type = dovecot
	smtpd_sasl_path = private/auth
	smtpd_sasl_security_options = noanonymous
	smtpd_sasl_tls_security_options = $smtpd_sasl_security_options
	smtpd_sasl_local_domain = $myhostname
	broken_sasl_auth_clients = yes
	smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
	smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
	smtpd_use_tls=yes
	smtpd_tls_auth_only = yes
	smtpd_tls_cert_file = /etc/ssl/private/server.crt
	smtpd_tls_key_file = /etc/ssl/private/server.key
	smtpd_tls_loglevel = 1

Let’s enable TLS on SMTP, modify /etc/postfix/master.cf like this:
	submission inet n       -       -       -       -       smtpd
	  -o syslog_name=postfix/submission
	  -o smtpd_tls_security_level=encrypt
	  -o smtpd_sasl_auth_enable=yes
	  -o smtpd_client_restrictions=permit_sasl_authenticated,reject

Created a new user vmail (with no shell access) to store emails:
	groupadd -g 5000 vmail
	useradd -u 5000 -g vmail -s /usr/bin/nologin -d /home/vmail -m vmail

And a new MySQL database/user to store info about domains and virtual users:
	CREATE DATABASE postfix_db;
	USE postfix_db;
	CREATE USER postfix_user@localhost IDENTIFIED BY 'postfix_user_password';
	GRANT ALL ON postfix_db.* TO postfix_user@localhost;
	FLUSH PRIVILEGES;

Then create the tables that are going to store the info related to domains, forwardings and users:
	CREATE TABLE `domains` (
	  `domain` varchar(50) NOT NULL default "",
	  PRIMARY KEY  (`domain`),
	  UNIQUE KEY `domain` (`domain`)
	);

	CREATE TABLE `forwardings` (
	  `source` varchar(80) NOT NULL default "",
	  `destination` text NOT NULL,
	  PRIMARY KEY  (`source`)
	);

	CREATE TABLE `users` (
	  `email` varchar(80) NOT NULL default "",
	  `password` char(128) NOT NULL default "",
	  `quota` varchar(20) NOT NULL default '20971520',
	  `domain` varchar(255) NOT NULL default "",
	  UNIQUE KEY `email` (`email`)
	);

Then start adding domains (make sure they have A and MX records properly set) in the database:
	INSERT INTO `domains` VALUES ('virtualdomain.tld');

And the account(s) related to the domain(s):
	INSERT INTO `users` VALUES ('info@virtualdomain.tld', ENCRYPT('password', CONCAT('$6$', 'salt_text')), '20971520', 'virtualdomain.tld');

Create a self-signed cert:
	cd /etc/ssl/private/
	openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out server.key
	chmod 400 server.key
	openssl req -new -key server.key -out server.csr
	openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt
	chmod 444 server.crt

And then the files we were referring to in Postfix config:
/etc/postfix/virtual_mailbox_domains.cf
	user = postfix_user
	password = postfix_user_password
	hosts = localhost
	dbname = postfix_db
	table = domains
	select_field = domain
	where_field = domain

/etc/postfix/virtual_mailbox_maps.cf
	user = postfix_user
	password = postfix_user_password
	hosts = localhost
	dbname = postfix_db
	table = users
	select_field = concat(domain,'/',email,'/')
	where_field = email

/etc/postfix/virtual_alias_maps.cf
	user = postfix_user
	password = postfix_user_password
	hosts = localhost
	dbname = postfix_db
	table = forwardings
	select_field = destination
	where_field = source

Create an empty transport file:
	touch /etc/postfix/transport
	postmap /etc/postfix/transport


Now replace the default Dovecot config file (/etc/dovecot/dovecot.conf) with:
	protocols = imap
	auth_mechanisms = plain login
	passdb {
	driver = sql
	args = /etc/dovecot/dovecot-sql.conf
	}
	userdb {
	driver = sql
	args = /etc/dovecot/dovecot-sql.conf
	}

	service auth {
	unix_listener /var/spool/postfix/private/auth {
	group = postfix
	mode = 0660
	user = postfix
	}
	user = root
	}

	mail_home = /home/vmail/%d/%u
	mail_location = maildir:~

	ssl = yes

	ssl_cert = 

	Create the /etc/dovecot/dovecot-sql.conf file:

	driver = mysql
	connect = host=localhost dbname=postfix_db user=postfix_user password=postfix_user_password
	# You can also set it on PLAIN. You can use MD5-CRYPT too, of course you'll need to change MySQL "users" table and have password as ENCRYPT('password').
	default_pass_scheme = SHA512-CRYPT
	# Get the mailbox
	user_query = SELECT '/home/vmail/%d/%u' as home, 'maildir:/home/vmail/%d/%u' as mail, 5000 AS uid, 5000 AS gid, concat('dirsize:storage=',  quota) AS quota FROM users WHERE email = '%u'
	# Get the password
	password_query = SELECT email as user, password, '/home/vmail/%d/%u' as userdb_home, 'maildir:/home/vmail/%d/%u' as userdb_mail, 5000 as  userdb_uid, 5000 as userdb_gid FROM users WHERE email = '%u'
	# If using client certificates for authentication, comment the above and uncomment the following
	#password_query = SELECT null AS password, ‘%u’ AS user


	... and finally restart dovecot and postfix:

	service postfix restart && service dovecot restart


	Try to send a mail to one of your newly created accounts, it should end up in a subdirectory of /home/vmail related to the domain.

	To access from an email client you need to set these info instead:

	Ingoing:

	Host: your FQDN

	Port: 993

	Use SSL: Yes

	Use authentication: Yes

	Outgoing:

	Host: your FQDN

	Port: 587

	Use SSL: Yes

	Use authentication: Yes