The new Background Send feature makes Gmail faster. Earlier, users had to wait patiently for a couple or more seconds for Gmail to send their messages and if the number of mails were more, users had to wait for other tasks. The Background Send feature is available in Gmail Labs and would be rolled out to all users soon.
"To help give you that time back, there"s a new feature in Gmail Labs called Background Send. Once you turn it on from the Labs tab in Settings, you can get on with what you"re doing while Gmail quietly sends off your mail in the background. You can keep reading your inbox, compose new messages, chat with people - all the things you"d usually do. You can even send more than one message in the background at the same time," Google's Software Engineer, Patrick Doyle, said in a blogpost.
If any error occurred while sending mails, users will get a warning message that prompts them to go back and fix the issue or try again later. The 'Send error' message will stay around until the users decide to fix things, so they don"t have to stop whatever they"re doing right away.
Anyhow, users have to wait for their mail to finish sending before they close Gmail or shut down the computer. Otherwise their mails will be cancelled.