Charset Windows-1252 not properly supported

[akusei]

I've been sending emails to someone who uses Outlook 2010 and the Outlook Privacy Plugin. He has been able to decrypt my inline PGP messages with no issue until recently. We discovered that when the Charset is "Charset: utf-8" OPP can decrypt it with no problems at all but when the charset is "Charset: Windows-1252" OPP cannot decrypt it and he ends up needing to decrypt it outside of outlook.

Any idea why this is happening?