Sometimes the e-mail client on MAC computers tends to stall. The best way to fix this issue is to restart it.

However, you do not need to restart the entire computer. If the program has stalled, or seems to be working very slow, choose Window: Activity. This will display a list of the tasks that the Mail application is performing.

Just click on the stop-sign icon next to your task to stop it. Once you have done that, you can start your e-mail client again.