Hotmail has a bad habit of breaking some email headers, when those headers are utf8-encoded.
Campaign Monitor describes it in detail. After some testing, I found out that the problem only occurs with relatively long headers. For example, an email with (random) subject “婰婜孲 訬軗郲, 榃痯痻 熿熼燛 鷢黳鼶 誙 慛慖 羳蟪蠁 湹渵焲 箛箙 瑽” displays incorrectly in Hotmail’s preview pane.
This particular subject translates to this header in the message’s source (the header was encoded using quoted-printable encoding):

Subject: =?utf-8?Q?=E5=A9=B0=E5=A9=9C=E5=AD=B2_=E8=A8=AC=E8=BB=97=E9=83=B2,_=E6=A6=83=E7=97=AF=E7=97=BB_=E7=86=BF=E7=86=BC=E7=87=9B_=E9=B7=A2=E9=BB=B3=E9=BC=B6_=E8=AA=99_=E6=85=9B=E6=85=96_=E7=BE=B3=E8=9F=AA=E8=A0=81_=E6=B9=B9=E6=B8=B5=E7=84=B2_=E7=AE=9B=E7=AE=99_=E7=91=BD?=

This header displays like this in Hotmail:

Now, how do we fix this (in other words: how do we work around Hotmail’s bug)?

Instead of using the quoted-printable encoding, you should use base64 encoding:

Subject: =?utf-8?B?5amw5amc5a2yIOiorOi7l+mDsiwg5qaD55ev55e7IOeGv+eGvOeHmyDpt6Lpu7PpvLYg6KqZIOaFm+aFliDnvrPon6rooIEg5rm55ri154SyIOeum+eumSDnkb0=?=

This results in correct display in Hotmail:

In summary: do not use quoted-printable encoding for email headers when you want them to be eligable in Hotmail, use base64 encoding instead.