I've only rarely used the "undo send" feature in GMail, but I'm always glad it's there in case I need it. Always thought AquaMail had the feature also. But now I see it doesn't. So I agree it should have such a feature.
Can't be too hard to do. Would work just like the Undo Delete feature, but you'd also be able to customize in settings the length of the delay — 5 secs, 10, 15, 30, whatever.
Or a pop-up with the Undo Send option could also include an option to Confirm Send or Send Now. The Send Now could be helpful for the times you're doing quick back and forth emailing with someone (almost like texting or chatting) and you don't want a delay.
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