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.
You should see the subject line and other header information of these older messages, but the body downloads only when you open the message, and the attachment downloads only when you open the attachment. You may not notice any delay if you have a fast Internet connection.
To fully download older messages and attachments without having to open them, either plug your Mac into AC power or follow these steps:
- Locate the "Paused (On Battery)" status message in the lower-left corner of the Mail window, beneath your list of mailboxes. If your mailboxes aren't visible, choose View > Show Mailbox List.
- Click the reload button next to the status message.