[sqwebmail] "setuid/setgid: Operation not permitted" when sending message

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I can see folders, read mails in INBOX and change password etc.

But when I try to send an email through "Create Message", it
says "setuid/setgid: Operation not permitted"

I read that webmail will invoke sendit.sh to send the mail, so I checked
that script and found that it will handle the mail to sendmail.

I can use sendmail to send mail without trouble, so I guess the problem is
between webmail and the sendit.sh script, but I am unable to figure out the
cause of this permission error.

Any help would be appreciated.

Some additional info:

I installed zdkimfilter, but it is not activated.

*permissions of sendmail and sendit.sh*
root@fancy-hat-4:/usr/lib/courier/bin# ls -l | grep sendmail
lrwxrwxrwx 1 root   root        8 Jul 20 13:26 rmail -> sendmail
-r-x--x--x 1 root   daemon  56424 Jul 20 13:26 sendmail

root@fancy-hat-4:/usr/lib/courier/share/sqwebmail# ls -l | grep sendit.sh
-rwxr-xr-x 1 bin    bin      875 Jul 20 13:26 sendit.sh

*ps -ef | grep courier*
root         523       1  0 Jul29 ?        00:00:00 /usr/sbin/courierlogger
-pid=/var/spool/authdaemon/pid -start
/usr/libexec/courier-authlib/authdaemond
root         524     523  0 Jul29 ?        00:00:00
/usr/libexec/courier-authlib/authdaemond
root         541     524  0 Jul29 ?        00:00:00
/usr/libexec/courier-authlib/authdaemond
root         542     524  0 Jul29 ?        00:00:00
/usr/libexec/courier-authlib/authdaemond
root         543     524  0 Jul29 ?        00:00:00
/usr/libexec/courier-authlib/authdaemond
root         544     524  0 Jul29 ?        00:00:00
/usr/libexec/courier-authlib/authdaemond
root         545     524  0 Jul29 ?        00:00:00
/usr/libexec/courier-authlib/authdaemond
daemon      9581       1  0 01:07 ?        00:00:00
/usr/lib/courier/libexec/courier/courierfilter start
daemon      9583       1  0 01:07 ?        00:00:00 /usr/sbin/courierlogger
courierfilter
root        9594       1  0 01:07 ?        00:00:00 /usr/sbin/courierlogger
-pid=/var/spool/courier/tmp/sqwebmaild.pid -start
/usr/lib/courier/libexec/courier/sqwebmaild
root        9595    9594  0 01:07 ?        00:00:00
/usr/lib/courier/libexec/courier/sqwebmaild
root        9605       1  0 01:07 ?        00:00:00
/usr/lib/courier/libexec/courier/sqwebmaild
root        9611       1  0 01:07 ?        00:00:00
/usr/lib/courier/libexec/courier/courierd
daemon      9613    9611  0 01:07 ?        00:00:00
/usr/lib/courier/libexec/courier/courierd
uucp        9615    9613  0 01:07 ?        00:00:00 ./courieruucp
root        9616    9613  0 01:07 ?        00:00:00 ./courierlocal
root        9617    9613  0 01:07 ?        00:00:00 ./courierfax
daemon      9618    9613  0 01:07 ?        00:00:00 ./courieresmtp
daemon      9619    9613  0 01:07 ?        00:00:00 ./courierdsn
daemon      9626       1  0 01:07 ?        00:00:00
/usr/lib/courier/sbin/couriertcpd -stderrlogger=/usr/sbin/courierlogger
-user=daemon -group=daemon -address=0.0.0.0
-access=/etc/courier/smtpaccess.dat -maxprocs=40 -maxperc=5 -maxperip=5
-pid=/var/spool/courier/tmp/esmtpd.pid smtp
/usr/lib/courier/sbin/courieresmtpd
daemon      9629       1  0 01:07 ?        00:00:00 /usr/sbin/courierlogger
courieresmtpd
root        9639       1  0 01:07 ?        00:00:00 /usr/sbin/courierlogger
-pid=/var/spool/courier/tmp/imapd.pid -start -name=imapd
/usr/lib/courier/sbin/couriertcpd -address=0 -maxprocs=40 -maxperip=4
-access=/etc/courier/imapaccess.dat -nodnslookup 143
/usr/lib/courier/libexec/courier/imaplogin /usr/lib/courier/bin/imapd
Maildir
root        9640    9639  0 01:07 ?        00:00:00
/usr/lib/courier/sbin/couriertcpd -address=0 -maxprocs=40 -maxperip=4
-access=/etc/courier/imapaccess.dat -nodnslookup 143
/usr/lib/courier/libexec/courier/imaplogin /usr/lib/courier/bin/imapd
Maildir
root        9651       1  0 01:07 ?        00:00:00 /usr/sbin/courierlogger
-pid=/var/spool/courier/tmp/imapd-ssl.pid -start -name=imapd-ssl
/usr/lib/courier/sbin/couriertcpd -address=0 -maxprocs=40 -maxperip=4
-access=/etc/courier/imapaccess.dat -nodnslookup 993
/usr/lib/courier/bin/couriertls -server -tcpd -user=daemon
/usr/lib/courier/libexec/courier/imaplogin /usr/lib/courier/bin/imapd
Maildir
root        9652    9651  0 01:07 ?        00:00:00
/usr/lib/courier/sbin/couriertcpd -address=0 -maxprocs=40 -maxperip=4
-access=/etc/courier/imapaccess.dat -nodnslookup 993
/usr/lib/courier/bin/couriertls -server -tcpd -user=daemon
/usr/lib/courier/libexec/courier/imaplogin /usr/lib/courier/bin/imapd
Maildir
root        9756       1  0 01:33 ?        00:00:00
/usr/lib/courier/libexec/courier/sqwebmaild
root        9776       1  0 01:33 ?        00:00:00
/usr/lib/courier/libexec/courier/sqwebmaild
root        9785       1  0 01:33 ?        00:00:00
/usr/lib/courier/libexec/courier/sqwebmaild
root        9792       1  0 01:33 ?        00:00:00
/usr/lib/courier/libexec/courier/sqwebmaild
root        9798    8132  0 01:41 pts/0    00:00:00 grep courier

*OS: Debian 12*

*installed packages related to courier:*
root@fancy-hat-4:/var/log# dpkg -l | grep courier
ii  courier                              1.3.10-100
amd64        Mail Server
ii  courier-imapd                        1.3.10-100
amd64        Courier Mail Server - IMAP server
ii  courier-ldap                         1.3.10-100
amd64        Courier Mail Server LDAP module and configuration screen
ii  courier-maildrop                     1.3.10-100
amd64        Courier Mail Server - mailrop mail filter
ii  courier-mlm                          1.3.10-100
amd64        Courier Mail Server - Mailing List Manager
ii  courier-mlm-web                      1.3.10-100
amd64        Courier Mail Server - Mailing List Manager (web interface)
ii  courier-mlm-web-apache2              1.3.10-100
amd64        Configure and enable apache for courier-mlm-web
ii  courier-mysql                        1.3.10-100
amd64        Courier Mail Server MySQL (MariaDB) configuration screen
ii  courier-webmail                      1.3.10-100
amd64        Courier Mail Server - HTTP (webmail) server
ii  libcourier-auth                      0.72.1-100
amd64        Courier authentication library
ii  libcourier-auth-config-daemon-daemon 0.72.1-100
amd64        Courier authentication library build configuration
ii  libcourier-auth-dev                  0.72.1-100
amd64        Courier authentication library (development and header files)
ii  libcourier-auth-mysql                0.72.1-100
amd64        Mysql (MariaDB) support for the Courier authentication library
ii  libcourier-auth0                     0.72.1-100
amd64        Courier authentication library (client library)
ii  libcourier-unicode-dev               2.3.0-100
 amd64        Courier Unicode Library (header files and development
libraries)
ii  libcourier-unicode8                  2.3.0-100
 amd64        Courier Unicode Library

Files are downloaded from https://www.courier-mta.org/download.html

DEB packages are built according to instructions on
https://www.courier-mta.org/install.html