+1 (out of votes)

I would love to see a 10 second abortable send... That's just good UX. However, being able to set arbitrary delays on sending operations is even more important (think repressive regimes etc.)...

The outbox should then look like... RECIPIENT, SUBJECT, DELIVERY, ACTIONS, e.g.:
Fred, Proposal v3, 2016-05-01 (+3 days, 01:23:22) [pull] [edit]

Also useful would be a deadman's switch variant where pulling the mail doesn't remove it from the outbox but resets it's countdown.

@Nancy: I see where you are coming from and I agree that it would be a nice feature to have. However it would also mean that a calender was also have to be implemented otherwise planned emails can be a little tedious. Imagine having to check your friends birthday, type the email, cross-reference dates etc for say 12 people..

In my opinion I think that it would be better to have a customized calendar for that. :)

I would like to delayed delivery as well. For example, I could have a Happy Birthday message prepared and, if I know the extra time of that person's birth, have the message sent that very hour and minute. I could say happy birthday to them at the exact time of their birth no matter their age. :) Also, if you have written an emotional email you aren't sure you want to send and it is saved in your drafts folder, you can sleep on it. You only have to take action to stop its delivery if you change your mind the next day. Otherwise, you know it will be automatically.

Already an idea for this, vote here instead: https://protonmail.uservoice.com/forums/284483-feedback/suggestions/7158412-schedule-emails-in-advance

As a security measure, the ability to set up a time delayed email, could make a difference for those who live in fear because of oppressive regimes.

Yes please

Ability to select specific date/time for email to send, after that email will appear in outbox or drafts and will be send on specified date.
I will send email today and my son received it after 10 years.

I would like to see send delay or undo sending

I too would like to see a time specific send feature! Some of us work with individuals in different time zones.
And I don't think the dev's want us to prioritize features, just give suggestions, so chill out and enjoy what they've done for us.

Like in gmail a 15/30 second delay from when you click "Send" allowing you to change your mind and amend something in the email. You can click "Undo".. make your change and send again. Recipient would receive the email only once.

I would prefer a possibility to set a specific time when the email will be sent in the future as suggedted in another proposition below. i like to write emails in the evenings and sleep longer but have them sent in the morning.

With respect, you can always discipline yourself to do this. Save as draft and then send when you are sure. I think, by comparison, there are more pressing enhancements that might be made to this excellent service.