I found this could be helpful for many people looking to create multiple Plesk email accounts especially since we are getting a lot of migrations from the old Google Gsuite Legacy edition.

The script reuires a txt file where the emails must be listed 1 per line :

#!/bin/bash
while read user
do
plesk bin mail --create $user -passwd 12345@@ -mbox_quota 25000M -mailbox true
done < userfile.txt

The script may fail if your secuity standards for your passwords are too high so you may edit that 12345
@ if needed.

By default i have aligned this at 15000M or 15G to equal Legacy Google Workspace accounts.

I hope this helps