Over the last few days we have had reports of Mailplane beach balling and needing to be force quit. We have found that the cause seems to be due to an incompatibility for users who downloaded the developer preview version of Safari (7.1).
Apple accidentally released it in the Mac App Store (available to everyone) and your system probably applied the update automatically. To check to see if you are indeed running 7.1, open Safari > Click Safari > About Safari. You will see your version number there.
We suggest using Gmail through Safari or Chrome at the moment. We will see if we can do something on our end, though we are hoping Apple will issue an update which will correct the troubles, as we are not the only ones suffering with the issue.
If you are NOT running Safari 7.1 and are having some troubles, please email us either through the Help > Support & Feedback menu item or at email@example.com.
Thanks so much for your understanding and patience.
UPDATE (July 16): Apple issued an update through Mac App Store that reverts you back to Safari 7.0.5. Make sure to restart your Mac after running the update.