Gmail Labs has a new feature called "Background Send." Once you turned it on from the Labs tab in Settings, you can continue 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 etc. You can even send more than one message in the background at the same time.
If anything goes wrong (maybe you got that email address wrong, or maybe your connection had a hiccup), you'll see a warning message that prompts you to go back and fix the issue or try again later.
The "Send errror" message will stay around until you decide to fix things, so you don't have to stop whatever you're doing right away. The only catch is that you should wait for your mail to finish sending before you close Gmail or shut down your computer. If messages are still being sent in the background when you shut down, your messages are probably going to be lost. You'll know you're good to go when you see a message like this:
[Source: Gmail blog]