The phrase refers to the process of ensuring that email data is consistent across multiple devices and servers. For example, when a message is read on a smartphone, that status is replicated on a desktop computer and a webmail interface. This synchronization applies to various aspects of email management, including sent items, deleted messages, and folder structures.
Maintaining consistency across platforms enhances user experience and productivity. It eliminates the frustration of managing multiple, disparate email states. Historically, email was often accessed from a single point. The rise of mobile devices and webmail necessitated a more robust synchronization to keep email accessible and manageable regardless of the access point.