Gmail has introduced a new Labs feature called Background Send that allows you to send one (or more) emails in the background, i.e. you don't have to wait for them to be sent before you can do other things in Gmail. If there are any send errors, it will notify you, but you have to fix them (e.g., if sending failed due to a mistake by you like a typo in the email address) before you log out of Gmail, or those pending emails may be lost..
Here is the post about it on the official Gmail Blog:
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- Vasudev Ram - Dancing Bison Enterprises