If two or more Macs are editing the same data (like the subject line of an email) in the same project at the same time, then it's possible for a conflict to arise. When this happens, Direct Mail will automatically resolve the conflict by choosing the most-recent change and syncing that across all devices.
Because Direct Mail uses time to resolve conflicts, it's important that the Date & Time settings on your Mac are correct. By default, Macs automatically set their time and timezone settings correctly. If the settings on your Mac somehow fall behind, Direct Mail will automatically alert you to the problem.