What is "Paused (On Battery)" mean in Mail.app? I saw the message "Downloading Messages Paused (On Battery)" in Mail.app in El Capitan.

What does this mean? The MacBook was plugged in to the wall, not running on battery. What would cause downloading to stop/pause?
It may have paused while you were operating your MacBook on battery power. As per Apple support document, If Mail on your Mac downloads messages only when you open them:


  When your Mac notebook is running on battery power, Mail in OS X El Capitan conserves energy by pausing the full download of messages that are that are older than 30 days, or older than the 10,000th message downloaded. Mail downloads messages from newest to oldest.