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Postbox was founded and staffed by several former developers from Mozilla. The software was built using Mozilla's Gecko browser engine, and was initially based on Thunderbird, Mozilla's own e-mail software.[4]
Postbox 7 requires 8GB of RAM, 200 MB of hard drive space, plus Windows 8 (or later) or macOS 10.13 (or later). The Windows version runs in 32-bit-mode on 32-bit and 64-bit systems.[5]