EDIT this functionality is/will be available also in Exchange 2016 On Premises with CU7
This happen all the time, you are sending an email and by the time you hit the Send button, you realize you have sent it to the wrong recipient(s).
With Exchange we all know (or should know) the recall feature but there is an important limitation: this works ONLY if the recipient(s) is/are hosted in the same Exchange organization and the message is not yet read.
So, what do we do if the message has been already processed and sent out; you try the recall (which means the recipient will receive the automatic message saying your tried to recall) but it will failed anyway to remove/delete the message.
Now, with Office 365 / Exchange Online you can undo a sent email for up to 30 seconds. Which means the sending action will be delayed up to the period defined.
To enable this new feature, logon to Outlook on the Web (formerly known as Outlook Web Access) and go to your settings to reach the Mail\Automatic Processing\Undo sent and select the time to allow the cancellation.
Once you have activated the Undo sent feature, next time you will send an email, the mail will not be sent immediately, instead it will be saved in the Draft folder for the period you have defined and only once the delay has been reached it will be sent.
This means if you want to retract this mail, you just have to go to your Draft folder and hit the Undo action button shown on top right of the folder view, which then will convert the mail to a draft.
IMPORTANT this works ONLY from Outlook on the Web