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Postbox on the Mac requires macOS 10.10 and later, and hardware that is capable of running 64-bit applications.
To determine if your Mac can run 64-bit applications, launch a program called "Activity Monitor" located within Applications / Utilities.
Under the "Kind" column, if you see applications running as "Intel (64-bit)," then you can run Postbox 2.5 or higher. If you only see "Intel," then you can only run Postbox 2.1.4 or earlier. A 32-bit version of Postbox 2.1.4 can be found here: