I follow the mantra "Inbox <20". Inbox 0 is not flexible enough and freestyle Inbox is not manageable in the long term.
Together with filters, freely reporting as spam/unsubscribing, my Inbox <20 becomes a sort of todo list which I can review and handle whenever needed (this include flight/hotel bookings, getting back to complex emails, etc.).
When I was a prof, I used to get a lot of emails. My system was that every few hours I would go through all new emails in my inbox and archive all of them in the following fashion
- Some emails required a one line immediate response. I did that.
- Some emails required a longer reply. I tagged them (in Thunderbird) as ToDo and archived them.
- Some emails had information I would need at a later meeting. Tagged as TempInfo and archived.
- Most emails were read once and archived.
Now, inbox is zeroed. Next, can attack items in ToDo one by one, and untag them, so the ToDo list is always short. Similarly, as soon as the relevant meeting finished, untag TempInfo emails.
Now, I work somewhere where Slack is used, resulting in an endless deluge of messages that cannot be controlled.