New in Gmail's Labs section is a feature that's a saving grace for anybody waiting out a big attachment or slow server. Once activated, "Background Send" lets you move on to reading and managing other messages, while Gmail continues sending your messages behind the scenes.
Head to the Labs tab in Settings, then search out "Background send." Enable that feature, and now your messages are sent using a different process, so you can start writing another message, or read through the rest of your inbox. If a message ultimately fails to send, Gmail will ping you at the top of your window to try it again now, or try it a bit later. It's a must-have Labs feature for anyone who needs reliable access to Gmail.
Gmail does note at the Labs switch that you need to stay logged into Gmail while your messages are sending in the background—likely not an issue in most cases, but worth keeping in mind.
New in Labs: Background Send [Official Gmail Blog]