In the Compose tab of Gmail, as you start typing out in the 'To' field, you are prompted with probable email addresses. It's convenient, no doubt, especially when you can't recall the email ID. But it could lead to an embarrassment, when in a hurry you end up sending the mail to the wrong person.
Gmail has an option, wherein within a specified period of time, you can recall the mail you sent. To enable this useful tool, click on the 'Gear' symbol on the top right of the email page, and go to 'Settings'. Click on 'Labs' in the series of options you see on the top of the page.
Scroll down and look for the 'Undo Send' option. Click Enable. Scroll down and click on 'Save changes'.
Click on 'General' in the top row of options. Scroll down and look for the 'Undo Send' option. Check 'Enable Undo Send', and choose from the options for cancellation period. The minimum is 5 seconds and the maximum is 30 seconds. Scroll down and Save Changes. Now, when you send a mail, it wouldn't go for the period of time you have selected, within which you can Undo Send.