Port 143 is not enabled by default, please use port 993 (IMAP over TLS) instead.

Directory /var/vmail must be owned by "vmail:vmail", permission 0700.

Try to enable insecure IMAP connection by following this tutorial, then try again with port 143:
http://www.iredmail.org/wiki/index.php? … t.STARTTLS

Need assistance
I can log in to webmail using Samba 4 users, but the problem is with trying to send an email message:
1) auto-complete of To: doesnt work: - "Dec 10 21:51:42 mail roundcube: PHP Error: Addressbook source (ldap_global) not found! in /usr/share/apache2/roundcubemail-0.8.2/program/include/rcmail.php on line 463 (POST /mail/?_task=mail&_action=autocomplete?_task=&_action=)"
2) Dec 10 18:07:15 mail postfix/smtpd[2652]: NOQUEUE: reject: RCPT from localhost[127.0.0.1]: 550 5.1.1 <vmail@example.com>: Recipient address rejected: User unknown in virtual mailbox table; from=<user1@example.com> to=<vmail@example.com> proto=ESMTP helo=<192.168.88.6>.
Any ideas where to look for?

There is an alias already in aliases - vmail: root

In /usr/share/apache2/roundcubemail/config/main.inc.php i have:
$rcmail_config['ldap_public']["example.com"] = array(
    'name'          => 'Global Address Book',
etc.
Still the same error messages!

Sorry - my bad

But have no idea what to do with this postfix error
Dec 11 17:19:01 mail roundcube: LDAP Error: Bind failed for dn=mail=user1@example.com,ou=Users,domainName=example.com,o=domains,dc=domain,dc=example,dc=com: Invalid credentials in /usr/share/apache2/roundcubemail-0.8.2/program/include/rcube_ldap.php on line 433 (GET /mail/?_task=mail&_action=list-contacts&_source=ldap_global&_page=1&_remote=1&_unlock=loading1355239152427&_=1355239152430)

Address book related part looks like this!
ldap_public in webmail is usable and works!
/usr/share/apache2/roundcubemail/config/main.inc.php
$rcmail_config['ldap_public']["ldap_global"] = array(
    'name'          => 'Global LDAP Address Book',
    'hosts'         => array('127.0.0.1'),
    'port'          => 389,
    'use_tls'       => false,
    'ldap_version'  => '3',
    'user_specific' => true, // If true the base_dn, bind_dn and bind_pass default to the user's IMAP login.

    // Search accounts in the same domain.
    'base_dn'       => 'domainName=%d,o=domains,dc=domain,dc=example,dc=com',
    'bind_dn'       => 'mail=%u@%d,ou=Users,domainName=%d,o=domains,dc=domain,dc=example,dc=com',
    'hidden'        => false,
    'searchonly'    => false,
    'writable'      => false,
    'search_fields' => array('mail', 'cn', 'sn', 'givenName', 'street', 'telephoneNumber', 'mobile', 'stree', 'postalCode'),

    // mapping of contact fields to directory attributes
    //   for every attribute one can specify the number of values (limit) allowed.
    //   default is 1, a wildcard * means unlimited
    'fieldmap' => array(
       // Roundcube  => LDAP:limit
        'name'        => 'cn',
        'surname'     => 'sn',
        'firstname'   => 'givenName',
        'title'       => 'title',
        'email'       => 'mail:*',
        'phone:work'  => 'telephoneNumber',
       'phone:mobile' => 'mobile',
        'street'      => 'street',
        'zipcode'     => 'postalCode',
       //'region'      => 'st',
        'locality'    => 'l',
        'department'  => 'departmentNumber',
        'notes'       => 'description',
        // these currently don't work:
        //'phone:workfax' => 'facsimileTelephoneNumber',
        //'photo'        => 'jpegPhoto',
        //'organization' => 'o',
        //'manager'      => 'manager',
        //'assistant'    => 'secretary',
    ),
    'sort'          => 'cn',
    'scope'         => 'sub',
    'filter'        => '(&(enabledService=mail)(enabledService=deliver)(enabledService=displayedInGlobalAddressBook)(|(objectClass=mailList)(objectClass=mailAlias)(objectClass=mailUser)))',
    'fuzzy_search'  => true,
    'vlv'           => false,   // Enable Virtual List View to more efficiently fetch paginated data (if server supports it)
    'sizelimit'     => '0',     // Enables you to limit the count of entries fetched. Setting this to 0 means no limit.
    'timelimit'     => '0',     // Sets the number of seconds how long is spend on the search. Setting this to 0 means no limit.
    'referrals'     => false,  // Sets the LDAP_OPT_REFERRALS option. Mostly used in multi-domain Active Directory setups
);