You log that next action in your system and move on to the next one, rather than doing the action t then and there (unless it’s super quick to do so).
By separating these two, you minimise mindset switching, and can be far more assured that you’re working on the right thing at any given time.
And of course, both feel obvious in retrospect. I don't even remember if there is anything else in GTD, but the value I get from those is enormous.
I'll respond to interesting email that comes in during the day if I feel like it / if I'm bored, but knowing that once a day I do a complete clear-out is helpful to me
anything in the inbox is something to action, anything you don't need to action personally, archive / tag or move to a folder and be done with it.
If it's something to action then at least gmail has a snooze button and I've found that to be super helpful.
My rule of thumb is that I should not have more than 30 items in my inbox. If there's more then clearing the inbox takes priority over anything else (bar key meetings).
I worked on a job where this was done and it worked flawlessly (emails generated from various systems and people, including those in the shift before you).
The one thing you have to make sure that your "job" communication is divided from your private communication or other non-actionable messages like newsletters etc.
Writing Emails to your self is totally okay also : )
I now check it ~2x. Morning and midday. If I'm doing a back and forth I may go back to that one specific thread, but that's it.
If you have too many tasks that needs to be done I think you should group them by activity and store them in a separate place. Then you can timebox "dealing with $activity" where you just get down with those tasks.
Eventually you can hire someone and give them the task of a certain activity (which can be anything: a software project, dealing with a visa application, buying a house etc)