It's assumed that you have already installed and configured Postfix according to this tutorial:
Installing Postfix with MySql backend and SASL for SMTP authentication

1. Install required packages

apt-get install dovecot-imapd dovecot-pop3d


2. Configure Dovecot (pico /etc/dovecot/dovecot.conf)

protocols = imap imaps pop3 pop3s
ssl_disable = no
mail_location = maildir:/home/vmail/%d/%n
auth default {
passdb sql {
args = /etc/dovecot/dovecot-sql.conf
}
userdb sql {
args = /etc/dovecot/dovecot-sql.conf
}
}
disable_plaintext_auth = no
user = vmail


Add or edit the above properties and leave everything else with the default value or configure according to your needs

3. Configure the MySql connector (pico /etc/dovecot/dovecot-sql.conf)

driver = mysql
connect = host=127.0.0.1 dbname={database} user={username} password={password}
default_pass_scheme = CRYPT
password_query = SELECT email as user, password FROM users WHERE email = '%u'
user_query = SELECT CONCAT('/home/vmail/',SUBSTRING_INDEX(email,'@',-1),'/',SUBSTRING_INDEX(email,'@',1),'/') as home, '5000' as uid, '5000' as gid FROM users WHERE email = '%u'


{database} = MySql database name
{username} = MySql username
{password} = MySql password

4. Restart Dovecot daemon

/etc/init.d/dovecot restart