Sounds a lot like my problem, which I described some days ago here on the list, but no luck. Please report to the list if you make any progress.

Regards,

Hi, I hope you're all right. I describe below the scenario where the
problem occurs.
I'm trying to activate a master user [1] to be able to access all the
boxes of all users by imap.
I have configured the dovecot-master-users [2] file with the
appropriate permissions.
When I try to access, for example with roundcube, through
user@mydomain.com*my_master_user@not-exist.com
I see in the log, that it seems to ignore the master-user (*) and
tries to authenticate the Active Directory but with the master user's
password. Then it shows in the log the messages

auth: Info: ldap(user@mydomain.com,127.0.0.1,<6sFGXE6XLwB/AAAB> )
invalid credentials
auth: Debug: client passdb out: FAIL 1 user=user@mydomain.com

I have other test cases, but I need to validate the master user login that way.

I would appreciate any comments you can make and I am waiting to
answer any questions.

Regards,

[1] https://doc.dovecot.org/configuration_manual/authentication/master_users/
[2] https://docs.iredmail.org/dovecot.master.user.html

---------------------------------------------------------------------------------------

root@mail-01:/etc/dovecot# cat /etc/issue
Ubuntu 14.04.5 LTS \n \l

root@mail-01:/etc/dovecot# dpkg -l | grep dovecot | awk {'print $2" "$3'}
dovecot-core 1:2.2.9-1ubuntu2.6
dovecot-imapd 1:2.2.9-1ubuntu2.6
dovecot-ldap 1:2.2.9-1ubuntu2.6
dovecot-lmtpd 1:2.2.9-1ubuntu2.6
dovecot-managesieved 1:2.2.9-1ubuntu2.6
dovecot-mysql 1:2.2.9-1ubuntu2.6
dovecot-pop3d 1:2.2.9-1ubuntu2.6
dovecot-sieve 1:2.2.9-1ubuntu2.6

root@mail-01:/etc/dovecot# cat dovecot.conf | grep -v '#' | grep -v -e
'^$' | sed  "s/REALDOMAIN/mydomain/g"
listen = * [::]
mail_plugins = quota mailbox_alias acl
protocols = pop3 imap sieve lmtp
mail_uid = 2000
mail_gid = 2000
first_valid_uid = 2000
last_valid_uid = 2000
log_path = /var/log/dovecot.log
auth_verbose = yes
auth_debug = yes
ssl_protocols = !SSLv2 !SSLv3
ssl = required
verbose_ssl = no
ssl_ca = </etc/ssl/certs/mydomain.com.ca-bundle
ssl_cert = </etc/ssl/certs/mydomain.com.crt
ssl_key = </etc/ssl/private/mydomain.com.key
ssl_cipher_list =
ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:ECDHE-RSA-DES-CBC3-SHA:ECDHE-ECDSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA
ssl_dh_parameters_length = 2048
ssl_prefer_server_ciphers = yes
disable_plaintext_auth = no
mail_location = maildir:%Lh/Maildir/:INDEX=%Lh/Maildir/
auth_default_realm = mydomain.com
auth_mechanisms = PLAIN LOGIN
service auth {
    unix_listener /var/spool/postfix/private/dovecot-auth {
        user = postfix
        group = postfix
        mode = 0666
    }
    unix_listener auth-master {
        user = vmail
        group = vmail
        mode = 0666
    }
    unix_listener auth-userdb {
        user = vmail
        group = vmail
        mode = 0660
    }
}
service lmtp {
    user = vmail
    process_min_avail = 5
    executable = lmtp -L
    unix_listener /var/spool/postfix/private/dovecot-lmtp {
        user = postfix
        group = postfix
        mode = 0600
    }
    inet_listener lmtp {
        address = 127.0.0.1
        port = 24
    }
}
auth_master_user_separator = *
passdb {
    driver = passwd-file
    args = /etc/dovecot/dovecot-master-users
    master = yes
}
userdb {
    args = /etc/dovecot/dovecot-ldap.conf
    driver = ldap
}
passdb {
    args = /etc/dovecot/dovecot-ldap.conf
    driver = ldap
}
plugin {
    auth_socket_path = /var/run/dovecot/auth-master
    quota = dict:user::proxy::quotadict
    quota_rule = *:storage=3G
    quota_warning = storage=100%% quota-warning 100 %u
    quota_warning2 = storage=95%% quota-warning 95 %u
    quota_warning3 = storage=90%% quota-warning 90 %u
    quota_warning4 = storage=85%% quota-warning 85 %u
    quota_grace = 10%%
    acl = vfile
    acl_shared_dict = proxy::acl
    sieve_dir = %Lh/sieve
    sieve = %Lh/sieve/dovecot.sieve
    sieve_global_dir = /var/vmail/sieve
    sieve_before = /var/vmail/sieve/dovecot.sieve
    mailbox_alias_old = Sent
    mailbox_alias_new = Sent Messages
    mailbox_alias_old2 = Sent
    mailbox_alias_new2 = Sent Items
}
service quota-warning {
    executable = script /usr/local/bin/dovecot-quota-warning.sh
    unix_listener quota-warning {
        user = vmail
        group = vmail
        mode = 0660
    }
}
service dict {
    unix_listener dict {
        mode = 0660
        user = vmail
        group = vmail
    }
}
dict {
    quotadict = mysql:/etc/dovecot/dovecot-used-quota.conf
    acl = mysql:/etc/dovecot/dovecot-share-folder.conf
}
protocol lda {
    mail_plugins = $mail_plugins sieve
    auth_socket_path = /var/run/dovecot/auth-master
    log_path = /var/log/dovecot-sieve.log
    lda_mailbox_autocreate = yes
    lda_mailbox_autosubscribe = yes
    postmaster_address = it@mydomain.com
}
protocol lmtp {
    info_log_path = /var/log/dovecot-lmtp.log
    mail_plugins = quota sieve
    postmaster_address = postmaster
    lmtp_save_to_detail_mailbox = yes
    recipient_delimiter = +
}
protocol imap {
    mail_plugins = $mail_plugins imap_quota imap_acl
    imap_client_workarounds = tb-extra-mailbox-sep
    mail_max_userip_connections = 30
}
protocol pop3 {
    mail_plugins = $mail_plugins
    pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
    pop3_uidl_format = %08Xu%08Xv
    mail_max_userip_connections = 30
}
service imap-login {
    service_count = 1
    process_limit = 500
}
service pop3-login {
    service_count = 1
}
service managesieve-login {
    inet_listener sieve {
        address = 127.0.0.1
        port = 4190
    }
}
namespace {
    type = private
    separator = /
    prefix =
    inbox = yes
    mailbox Sent {
        auto = subscribe
        special_use = \Sent
    }
    mailbox "Sent Messages" {
        auto = no
        special_use = \Sent
    }
    mailbox "Sent Items" {
        auto = no
        special_use = \Sent
    }
    mailbox Drafts {
        auto = subscribe
        special_use = \Drafts
    }
    mailbox Trash {
        auto = subscribe
        special_use = \Trash
    }
    mailbox "Deleted Messages" {
        auto = no
        special_use = \Trash
    }
    mailbox Junk {
        auto = subscribe
        special_use = \Junk
    }
    mailbox Spam {
        auto = no
        special_use = \Junk
    }
    mailbox "Junk E-mail" {
        auto = no
        special_use = \Junk
    }
    mailbox Archive {
        auto = subscribe
        special_use = \Archive
    }
    mailbox Archives {
        auto = no
        special_use = \Archive
    }
}
namespace {
    type = shared
    separator = /
    prefix = Shared/%%u/
    location = maildir:%%Lh/Maildir/:INDEX=%%Lh/Maildir/Shared/%%u
    subscriptions = yes
    list = children
}

root@mail-01:/etc/dovecot# cat dovecot-ldap.conf | sed
"s/realdomain/mydomain/g"
hosts           = active-directory:389
ldap_version    = 3
auth_bind       = yes
dn              = domain\vmail
dnpass          = Str0ngP4ssw0rd
base            = ou=Organizational Unit,dc=domain,dc=com
scope           = subtree
deref           = never
user_filter     =
(&(userPrincipalName=%n@domain.com)(objectClass=person)(!(userAccountControl:1.2.840.113556.1.4.803:=2)))
pass_filter     =
(&(userPrincipalName=%n@domain.com)(objectClass=person)(!(userAccountControl:1.2.840.113556.1.4.803:=2)))
#user_filter     =
(&(mail=%u)(objectClass=person)(!(userAccountControl:1.2.840.113556.1.4.803:=2)))
#pass_filter     =
(&(mail=%u)(objectClass=person)(!(userAccountControl:1.2.840.113556.1.4.803:=2)))
pass_attrs      = userPassword=password
default_pass_scheme = CRYPT
user_attrs      = \
                #=mail=master_user, \
#mail=user, \
=user=%Ln@mydomain.com, \
=home=/var/vmail/vmail1/mydomain.com/%Ln/Maildir/, \
=mail=maildir:/var/vmail/vmail1/mydomain.com/%Ln/Maildir/, \
maxStorage=quota_rule=*:bytes=%$


root@mail-01:/etc/dovecot# ls -al dovecot-master-users
-r-x------ 1 dovecot dovecot 120 Nov 13 14:45 dovecot-master-users