Debian 11 uses exim4 4.94 and it doesn’t support SRS, you should replace /etc/exim4/exim4.conf.template with /usr/local/hestia/install/deb/exim/exim4.conf.4.94.template (remember to backup your original conf) and restart exim4 once the conf file is replaced.
I forgot to say that if you are using spamassassin and/or clamav antivirus, you should uncomment the definitions in /etc/exim4/exim4.conf.template and restart again exim4.