dovecot   unix  -       n       n       -       -       pipe
  flags=DRhu user=email:email argv=/usr/lib/dovecot/deliver -f ${sender} -d ${recipient}


Finally, there is one last file that you need to edit out: /etc/postfix/sasl/smtpd.conf. If it doesn't exist, create it. Anyhow insert the follow values into the file:
[
pwcheck_method: saslauthd
mech_list: plain login cram-md5 digest-md5
]


Step 4: configuring Dovecot
It is now time to configure Dovecot, the program that will serve as POP3 and IMAP server. Open the dovecot main configuration file [/etc/dovecot/dovecot.conf] using your favorite text editor. As you can see this file is pretty big due to the massive amount of comments all over the place. I'm going to post the appropriate configuration below with my own comments pertaining to the architecture we're setting up. Feel free to copy paste it but make sure to modify the relevant sections.
[
# Basic configuration
protocols = imap imaps pop3 pop3s
log_timestamp = "%Y-%m-%d %H:%M:%S "
disable_plaintext_auth=yes

# User and group permissions
first_valid_uid=7788
last_valid_uid=7788
first_valid_gid=7788
last_valid_gid=7788
mail_location = maildir:/var/email/%d/%n/Maildir
mail_privileged_group = email
auth_executable = /usr/lib/dovecot/dovecot-auth
auth_verbose = yes

# SSL config
ssl_cert_file = /etc/ssl/private/example.com.cert
ssl_key_file = /etc/ssl/private/example.com.key

# LDA config
protocol lda {
  auth_socket_path = /var/run/dovecot/auth-master
  postmaster_address = postmaster@example.com
  mail_plugins = sieve
  log_path =
}

# Authentication configuration
auth default {
    mechanisms = plain login
    passdb passwd-file {
        args = scheme=SHA1 /etc/dovecot/users.conf
    }
    userdb static {
        #args = /etc/dovecot/users.conf
        args = uid=7788 gid=7788 home=/var/email/%d/%n allow_all_users=yes
    }
    socket listen {
        master {
            path = /var/run/dovecot/auth-master
            mode = 0600
            user = email
            group = email
        }
        client {
            path = /var/spool/postfix/private/auth
            mode = 0660
            user = postfix
            group = postfix
        }
    }
}
]

Next we need to create an empty users file, so create a blank file [/etc/dovecot/users.conf]. We will update it during the next step. To finish with this step, ensure that your configuration files have the proper permissions, by running the following commands:

[
chgrp email /etc/dovecot/dovecot.conf
chmod g+r /etc/dovecot/dovecot.conf
chown root:root /etc/dovecot/users.conf
chmod 600 /etc/dovecot/users.conf
]

Step 5: managing domains and mailboxes
At this point, our SMTP and IMAP/POP3 servers are properly configured. Now all we have to do is create mailboxes and user accounts for the domains of our choice. To begin with, we need to define the list of domains that are being handled by our server. Open up (by this I mean create or edit out)  [/etc/postfix/vmail_domains] and insert 1 domain per line, followed by a tabulation character, and the word OK. In our case:
example.com OK
example2.com OK

Now we're going to set up our mailboxes and accounts. There are three files involved, you'll need to edit them out every time you need to make changes to your mail accounts:
/etc/postfix/vmail_mailbox: contains the list of mailboxes along with their storage path
/etc/postfix/vmail_aliases: the list of email aliases
/etc/dovecot/users.conf: stores your encrypted mailbox passwords
Let's begin by vmail_mailbox. On each line you're supposed to enter the full e-mail address of the mailbox, a tabulation character, then the path of the mailbox files - relative to /var/email as we have indicated in the Dovecot and Postfix configurations. The directories you indicate here do not need to exist, they will be created automatically when needed; however they have to respect the syntax: domain.tld/user. My file contains:
webmaster@example.com    example.com/webmaster
contact@example.com   example.com/contact
webmaster@example2.com   example2.com/webmaster

Now, on to vmail_aliases. This is the alias list which can be used for email address forwarding or mailing lists. Each line must contain: the source address, a tabulation character, the destination address(es). Here's mine:
webmaster@example.com   webmaster@example.com
contact@example.com   contact@example.com
webmaster@example2.com   webmaster@example.com

And finally, [/etc/dovecot/users.conf]. This file contains the user accounts and encrypted passwords. Now how do we build that file knowing that passwords will be encrypted? Here's how: repeat the following process for each user/password combination you want to add:
Generate a password by running this command: [dovecotpw -s SHA1]. It will ask you to enter a password, and output the hashed result like this: {SHA1}qUqP5cyxm6YcTAhz05Hph5gvu9M=. Copy the part highlighted in yellow (the whole line except {SHA1}).
Open up users.conf and at the bottom of the file, add a new line respecting the following format: mailbox@example.com:password (copied from step 1)
My users.conf file now looks like this:
webmaster@example.com:qUqP5cyxm6YcTAhz05Hph5gvu9M=
contact@example.com:EJ9LPFDXsN9ynSmbxvjp75Bmlx8=
webmaster@example2.com:RTzySG+IxBH5rWCLVjrvllztsV0=

Now that you have updated your user database, it's time to apply the changes. Run the following commands for Postfix to acknowledge your newly created mailboxes:
[
postmap /etc/postfix/vmail_domains
postmap /etc/postfix/vmail_mailbox
postmap /etc/postfix/vmail_aliases

And now, it's time to start the Postfix and Dovecot services, making sure they're stopped first:
service postfix stop
service dovecot stop
service postfix start
service dovecot start
]
You can now try your new mailboxes with your favorite email client software. Here are the configuration settings that you may be requested to provide:
Server: example.com
User account: webmaster@example.com (full email address)
Password: the password you chose earlier when you ran the dovecotpw -s SHA1 command
Protocol: SMTP for sending (authentication required!), POP3 or IMAP for receiving
Ports: SMTP 25, SMTP secure 587, POP3 110, POP3 secure 995, IMAP 143, IMAP secure 993
You may need to properly configure the above ports in your server's firewall if you use one. If you have correctly performed all the steps and done as I said, there's no way this can go wrong. I have personally configured a good number of servers following these exact instructions, and it always worked great. Don't hesitate to post comments if you run into problems.