I'd welcome an outbound mail queue in gmail (or whatever web mail provider is your favorite). Preferably one with some configuration options. In my case, I might have a default delay of 2 minutes or so. For more uncertain messages, I might bump their individual delay out to some hours.

This allows for alteration of a message that has not yet made it off the queue. But it has the advantage over e.g. a draft message in that, if circumstances do not change, no further action is required.

If a queued email bothers you enough, you can go back in and change it. But you don't have to remember the things that end up not bothering you; they will be sent after their delay expires. This works in concert with the natural flow of your attention, rather than requiring you to keep track of and follow up on draft emails whose topics your mind has decided it is no longer worried about.

It can also be used to email yourself, or others, reminders at a pre-determined time.

I imagine one reason gmail, for example, does not add such a feature is all the potential avenues of abuse. Accounts set up with to spam without further sign on, or to harass someone else. Or just the person who ueber-automates every moment of their day with umpteen bazillion delayed messages in the latest GTD meme. Google may have better uses for its electrons.

Plus... If its automated, you're not looking at ads while it's sent.