This is from the release notes of 9.12:
PGP Desktop for Windows has been tested with the following email clients:
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Microsoft Outlook 2007 SP1 (Outlook 12)
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Microsoft Outlook 2003 SP3
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Microsoft Outlook XP SP3
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Microsoft Outlook 2000 SP3
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Microsoft Windows Mail 6.0.600.16386
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Microsoft Outlook Express 6 SP1
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Mozilla Thunderbird 2.0
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Lotus Notes 6.5.6, 7.0.3, 8.02, 8.5