How to reduce the mails to the minimum?
Every mail that is not relevant – just delete it.
If the mail has some relevance, but no action is needed, it might be a good idea to store it. If you really need it as a reference on the long term store it in some reference folder. If you just want to keep it because it might be useful in the next months, create a folder that automatically deletes content older than 6 months.
If action is needed, but you can delegate it, forward it to that person with a clear expectation/deadline and put it in some scheduled review folder – typically weekly reviewed.
In case it is my personal task, lets judge how long it might take. If it takes more than 2 minutes to work on the topic, lets do it in a dedicated blocked time-slot during the day for such tasks.
If action is needed by me personally and it can be done within 2 minutes – just do it and feel happy that it is done 😉