[imp] encoding problem

[yahsuan at Lab400-51.cc.ntu.edu.tw]

my horde app version is:
horde                     4.0.1   stable
imp                       5.0.1   stable
ingo                      2.0.1   stable
turba                     3.0.1   stable

and my php version is PHP 5.3.3

I find that when I send a plain text mail, the subject will by encoded,
but if I send a mail with attachment, this mail will be a mime type mail,
and the subject will no to be encoded.
This lead a problem when viewing.

Is this a bug in imp5.0.1?
Since the old version(horde3.3.4) will encode the subject no matter I send
a
plain text mail or a mime mail.

Thanks for your reply in advance.

Following is the extract from two mail, first is a plain text mail, and
second is a
mail with attachment.

1.
From: yahsuan at Lab400-51.cc.ntu.edu.tw
To: sumtiogo <sumtiogo at gmail.com>
Subject: =?big5?b?vXO9WA==?=
User-Agent: Internet Messaging Program (IMP) H4 (5.0.1)
Content-Type: text/plain; charset=BIG5; format=flowed; DelSp=Yes
MIME-Version: 1.0
Content-Disposition: inline

2.
From: yahsuan at Lab400-51.cc.ntu.edu.tw
To: sumtiogo <sumtiogo at gmail.com>
Subject: 編碼
User-Agent: Internet Messaging Program (IMP) H4 (5.0.1)
Content-Type: multipart/mixed; boundary="=_5h9pYbd-V21NwRXoYR2VoXA"
MIME-Version: 1.0

This message is in MIME format.

--=_5h9pYbd-V21NwRXoYR2VoXA
Content-Type: text/plain; charset=BIG5; format=flowed; DelSp=Yes
Content-Disposition: inline