Mailer=prog, pri=4044464, dsn=4.0.0, stat=Deferred: prog mailer (/bin/sh) exited with EX_TEMPFAIL

Your suggestions are highly appreciated.

My RT installation is up and running
Except the mail gateway

I get in my mail.log:

localhost sm-mta[181115]: 21EBihYO016697: to="|/opt/rt5/bin/rt-mailgate --queue general --action correspond --debug -url https://xuz.abc/", ctladdr=xyz@abc.pqr (8/0), delay=01:27:30, xdelay=00:00:02, mailer=prog, pri=847196, dsn=4.0.0, stat=Deferred: prog mailer (/bin/sh) exited with EX_TEMPFAIL

Restarting the server does not help.

I added --debug in the aliases and ran newalias. Then I was able to record the following issue in syslog:
[16894] Couldn’t create object file /opt/rt5/var/mason_data/obj/3595297236/standard/Admin/Groups/GroupRights.html.obj: Permission denied#012Stack:#012 [/usr/share/perl5/HTML/Mason/Interp.pm:437]#012 [/usr/share/perl5/HTML/Mason/Request.pm:250]#012 [/usr/share/perl5/HTML/Mason/Request.pm:212]#012 [/opt/rt5/sbin/…/lib/RT/Interface/Web/Request.pm:63]#012 [/usr/share/perl5/Class/Container.pm:275]#012 [/usr/share/perl5/Class/Container.pm:353]#012 [/usr/share/perl5/HTML/Mason/Interp.pm:348]#012 [/usr/share/perl5/HTML/Mason/Interp.pm:342]#012 [/usr/share/perl5/HTML/Mason/PSGIHandler.pm:59]#012 [/usr/share/perl5/HTML/Mason/PSGIHandler/Streamy.pm:52]#012 [/usr/local/share/perl/5.26.1/Plack/Util.pm:339]#012 [/usr/local/share/perl/5.26.1/Plack/Util.pm:339]#012 [/usr/local/share/perl/5.26.1/Plack/Handler/FCGI.pm:149]#012 [/usr/local/share/perl/5.26.1/Plack/Loader.pm:84]#012 [/usr/local/share/perl/5.26.1/Plack/Runner.pm:279]#012 [/opt/rt5/sbin/…/lib/RT/PlackRunner.pm:150]#012 [/opt/rt5/sbin/rt-server.fcgi:162]