Just found a built in python script for resetting passwords of multiple mail users with one go. This python script came with the installation of iRedmail.
You can find the python scrip at this path
All you have to do is to create a csv file in excel and put username+domain and password in plain text, as per below
Put the password in same cell with space and DO NOT put the password in another cell, otherwise the script wouldn't run.
Save the CSV file with any name, here we put the name as new_passwords.csv
Now upload the CSV to ired mail server on same path /var/www/iredadmin/tools/
Run the python scrip with the uploaded file with below command.
python /var/www/iredadmin/tools/update_password_with_csv.py new_passwords.csv
You have successfully reset the password for all required email user account.