iRedMail enables Dovecot plugin and service "quota_warning" by default, you can find settings in dovecot.conf like below:

plugins {
    ...
    # Quota warning.                                                               
    # If user suddenly receives a huge mail and the quota jumps from               
    # 85% to 95%, only the 95% script is executed.                                 
    quota_warning = storage=85%% quota-warning 85 %u                                                                          
    quota_warning2 = storage=90%% quota-warning 90 %u                              
    quota_warning3 = storage=95%% quota-warning 95 %u     
    ...
}

service quota-warning {
    executable = script /usr/local/bin/dovecot-quota-warning.sh
    ...
}

Please edit /usr/local/bin/dovecot-quota-warning.sh, insert a BCC header above "Subject:" line like below:

From: no-reply@demo.iredmail.org
BCC: postmaster@your_domain.com
Subject: Mailbox Quota Warning: ${PERCENT}% Full.

Your mailbox is now ${PERCENT}% full, please clean up some mails for
further incoming mails.

Let me know whether it works for you or not. If it doesn't work, you can also add similar command in dovecot-quota-warning.sh to send an email to admin.