Requested feature overview description.

A pop-up that shows up and asks you for confirmation once you press Send.

Why would it be useful?

Often, I check how a DMail I'm writing looks so far, and especially on phone, I sometimes slip and press send instead of Preview because the buttons are right next to each other.

This isn't the same as making a comment, blip or forum post, because you don't have the option to correct your mistake afterwards; the DMail has already been sent.

This feature would be especially useful for administration and users who DMail a lot.
Confirmation pop-ups might also serve well elsewhere on e621, mainly on administrative pages.

What part(s) of the site page(s) are affected?

DMails/Possibly other pages

I think it would be useful to have a confirmation. Also for those who may not want to confirm, there might be some sort of button you can click that disables it for a time or permanently toggle.

Has my vote and it'd be rather simple to add. Some might complain, but that's where the other part of the idea comes into play.

Aye, I give this a +1 too. I don't make the mistakes myself because I don't DMail often, but I do notice times where rephrasing what I said 'd be good.

One additional thing that could happen besides an onclick confirm "Are you sure?", maybe it also previews anyways allowing you to look over it one last time.

That could be useful.

As long as there's a toggle. Sure.