We sometimes get broken character encodings such as:
Subject: =?iso-2022-jp?B?GyRCLWo7M3l1OSk2SBsoQgo=?=
This actually is not a strict ISO-2022-JP string, but a CP50220 string
that is a variant of ISO-2022-JP with extended characters proposed by
Microsoft. Iconv can not handle these encodings well.