If you have Offline Gmail enabled, you'll notice that all your mail now goes through the outbox, regardless of whether you're online or offline. This allows Gmail to capture all attachments, even if you suddenly get disconnected from network. If you're online, your mail will quickly be sent along to its destination.
If you haven't tried offline access yet, visit the Labs tab and follow these instructions to get started:
- Select Enable next to Offline Gmail.
- Click Save Changes.
- After your browser reloads, you'll see a new "Offline" link in the upper righthand corner of the Gmail page, next to your username. Click this link to start the offline set up process and download Gears if you don't already have it.
Via Official Gmail Blog