The option to change your mind after you've sent an email from Gmail is one the most useful features for getting you out of a spot of bother, and it works both in the mobile apps and on desktop web browser.
Even if you've not yet sent a message you regret, you should check your Gmail settings now. That's because Undo Send works for only 5 seconds by default and you can increase this to 30 seconds.
Also note that Undo will only appear if you're sending from a Gmail account: the mobile app allows you to see other email accounts from other providers, but you can only recall an email when you sent it from a Gmail account.
Increase Undo Send Gmail to 30 seconds
You will need to be using a PC or laptop browser to complete this step, because even when you opt to view the Desktop version of Gmail in a mobile browser you will be shown the old version that does not contain the Undo Send feature.
So on a PC or laptop, launch your web browser and open Gmail. Tap the cog icon at the top right and choose Settings. On the General tab look for Undo Send. To the right of this you'll see 'Send cancellation period', by default set to 5 seconds, but you can tap on the drop-down menu to change this to 30 seconds. Now scroll down to the bottom of the page and click Save changes.
Do remember that if you increase this feature to the maximum length of time, no emails will leave your Outbox until 30 seconds have passed. So if people are complaining that the email you just sent them has not yet arrived, this is why.
How to Unsend Gmail on Android
When you compose an email message in the Gmail app on Android and then tap the Send button it will automatically return you to the conversation view.
Watch carefully the bottom of the screen: you'll see a black bar appear that says 'Sent' on the left, with 'Undo' on the right. (Again, remember that this will NOT appear if you sent the email from a non-Gmail account.)
Quickly tap Undo and you'll be returned to the message composition screen, where you can delete or edit the email as required.
Unsend Gmail on a PC or laptop
Recalling a Gmail message on a PC or laptop browser works much the same as it does on Android.
After you have hit Send, look to the bottom left of the screen for an option that says 'Message sent', with further options to Undo or View message.
Click Unsend and the message will pop up onscreen again, so you can edit or delete as required.
You might like to know how to disable Gmail Nudges as well.