I am writing my own IMAP client and I have noticed this issue.

I am getting this response when asking for the envelope of a message:

("Mon, 12 Jul 2010 18:18:25 +0100" "Polar coordinates" (("Gabriela Gonzalez Castro" NIL "gabriela.gonzalez" "tangentix.com")) (("<gonzalezcas...@googlemail.com>" NIL "=?ISO-8859-1?Q?\"Gabriela_Gonz=E1lez_Castro\"?=" NIL)) (("Gabriela Gonzalez Castro" NIL "gabriela.gonzalez" "tangentix.com")) ((NIL NIL "peter.jeffery" "tangentix.com")) NIL NIL NIL "<F7DDEFD7-6E18-4080-9A52-8A6C1E38CC3B@tangentix.com>")

so the sender is listed as (("<gonzalezcas...@googlemail.com>" NIL "=?ISO-8859-1?Q?\"Gabriela_Gonz=E1lez_Castro\"?=" NIL))

The text of the message header is :

Sender: =?UTF-8?Q?Gabriela_Gonz=C3=A1lez_Castro?= <gonzalezcas...@googlemail.com>
From: Gabriela Gonzalez Castro <gabriela...@tangentix.com>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: quoted-printable
Subject: Polar coordinates
Date: Mon, 12 Jul 2010 18:18:25 +0100
Message-Id: <F7DDEFD7-6E18-4080-9A52-8A6C1E38CC3B@tangentix.com>
To: peter....@tangentix.com
Mime-Version: 1.0 (Apple Message framework v1081)
X-Mailer: Apple Mail (2.1081)

So it looks to me like the sender should be (("=?ISO-8859-1?Q?\"Gabriela_Gonz=E1lez_Castro\"?=" NIL "gonzalezcastrogabriela" "googlemail.com" )) instead. It seems like the encoding of the sender name as confused the parser.