postinst is in mail-stack-delivery 1:2.2.9-1ubuntu2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 | #!/bin/sh
set -e
POSTFIX_BCKFILE="/var/backups/mail-stack-delivery/main.cf-backup"
set_postfix_option() {
    opt="$1"
    # Backup the existion value of the option
    postconf $(echo ${opt} | cut -d= -f1) >> ${POSTFIX_BCKFILE} || true
    # Set the new value of the option
    postconf -e "${opt}"
    echo -n '.'
}
if [ "$1" = "configure" ]; then
    # Create initial symlinks for certificates
    SSL_CERT=$( (grep -m 1 "ssl_cert_file" /etc/dovecot/conf.d/10-ssl.conf || echo '/etc/dovecot/dovecot.pem') | cut -d'=' -f2)
    SSL_KEY=$( (grep -m 1 "ssl_key_file" /etc/dovecot/conf.d/10-ssl.conf || echo '/etc/dovecot/private/dovecot.pem') | cut -d'=' -f2)
    if [ ! -e "${SSL_KEY}" ]; then
        ln -s /etc/ssl/private/ssl-cert-snakeoil.key ${SSL_KEY}
    fi
    if [ ! -e "${SSL_CERT}" ]; then
        ln -s /etc/ssl/certs/ssl-cert-snakeoil.pem ${SSL_CERT}
    fi
    # Configure postfix either on new install 
    # or if the postfix backup file is no longer there 
    # (only deleted when the pkg is removed)
    if [ -f "/etc/postfix/main.cf" ]; then
        if [ -e "$POSTFIX_BCKFILE" ]; then
           cp $POSTFIX_BCKFILE ${POSTFIX_BCKFILE}-$(date +%Y%m%d%H%M)
        fi
        if [ -z "$2" -o ! -e "$POSTFIX_BCKFILE" ]; then
            if which postconf >/dev/null; then
                # Setup postfix
                echo 'Mail stack delivery changes some postfix settings.'
                echo 'Old values are stored in '$POSTFIX_BCKFILE'.'
                echo 'Feel free to revert any of them when the process is done.'
                echo -n 'Configuring postfix for mail-stack-delivery integration: '
                set_postfix_option "home_mailbox = Maildir/"
                set_postfix_option "smtpd_sasl_auth_enable = yes"
                set_postfix_option "smtpd_sasl_type = dovecot"
                set_postfix_option "smtpd_sasl_path = private/dovecot-auth"
                set_postfix_option "smtpd_sasl_authenticated_header = yes"
                set_postfix_option "smtpd_sasl_security_options = noanonymous"
                set_postfix_option "smtpd_sasl_local_domain = \$myhostname"
                set_postfix_option "broken_sasl_auth_clients = yes"
                set_postfix_option "smtpd_recipient_restrictions = reject_unknown_sender_domain, reject_unknown_recipient_domain, reject_unauth_pipelining, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination"
                set_postfix_option "smtpd_sender_restrictions = reject_unknown_sender_domain"
                set_postfix_option "mailbox_command = /usr/lib/dovecot/deliver -c /etc/dovecot/dovecot.conf -m \"\${EXTENSION}\""
                set_postfix_option "smtpd_tls_cert_file = ${SSL_CERT}"
                set_postfix_option "smtpd_tls_key_file = ${SSL_KEY}"
                set_postfix_option "smtpd_use_tls = yes"
                set_postfix_option "smtp_use_tls = yes"
                set_postfix_option "smtpd_tls_received_header = yes"
                set_postfix_option "smtpd_tls_mandatory_protocols = SSLv3, TLSv1"
                set_postfix_option "smtpd_tls_mandatory_ciphers = medium"
                set_postfix_option "smtpd_tls_auth_only = yes"
                set_postfix_option "tls_random_source = dev:/dev/urandom"
                echo ' done.'
            fi
        fi
        # Parameters that need to be changed on upgrades
        if [ ! -z "$2" ] && dpkg --compare-versions $2 lt 1:2.1.7-7ubuntu1; then
            set_postfix_option "mailbox_command = /usr/lib/dovecot/deliver -c /etc/dovecot/dovecot.conf -m \"\${EXTENSION}\""
        fi
    else
        echo ""
        echo "Postfix not configured. Run"
        echo "sudo dpkg-reconfigure postfix and choose"
        echo "the type of mail server. Then run"
        echo "sudo dpkg-reconfigure mail-stack-delivery to"
        echo "finish mail-stack-delivery installation."
        echo ""
    fi
    if [ -x "/etc/init.d/dovecot" ]; then
        if [ -x /usr/sbin/invoke-rc.d ]; then
            invoke-rc.d dovecot restart
        else
            service dovecot restart
        fi
    fi
    if [ -x "/etc/init.d/postfix" ]; then
        if [ -x /usr/sbin/invoke-rc.d ]; then
            invoke-rc.d postfix restart
        else
            service postfix restart
        fi
    fi
fi
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