Yahoo marking spam only from mutt

I have mutt set up to send email for my Gmail account. .muttrc looks like this:

account-hook imaps://imap.gmail.com:993 'set imap_user="me@gmail.com"'
folder-hook imaps://imap.gmail.com:993 'set \
    folder="imaps://imap.gmail.com:993" \
    spoolfile="+INBOX" \
    copy=no \
    postponed="+[Gmail]/Drafts" \
    from="Me <me@gmail.com>" \
    smtp_url="smtps://me\@gmail.com@smtp.gmail.com"'

When I send an email to a test Yahoo account from mutt, it ends up in the spam folder. When I send an email to the same account from either the Gmail web interface or Mail.app on OS X, the message lands in the Yahoo inbox. These are the only differences I can discern in the headers:

MUTT

Received: by mail-gy0-f175.google.com with SMTP id 10so3753273gyg.6
        for <testaccount@yahoo.com>; Tue, 15 Jun 2010 12:24:58 -0700 (PDT)

Received: from mydomain.com (mydomain.com [0.0.0.0])
        by mx.google.com with ESMTPS id m39sm31085319ann.1.2010.06.15.12.24.57
        (version=SSLv3 cipher=RC4-MD5);
        Tue, 15 Jun 2010 12:24:57 -0700 (PDT)

Message-ID: <20100615192453.GA21268@mydomain.com>

MAIL.APP

Received: by ywh3 with SMTP id 3so3930412ywh.31
        for <testaccount@yahoo.com>; Tue, 15 Jun 2010 12:41:25 -0700 (PDT)

Received: from [0.0.0.0] ([0.0.0.0])
        by mx.google.com with ESMTPS id j28sm20367390qck.10.2010.06.15.12.41.22
        (version=TLSv1/SSLv3 cipher=RC4-MD5);
        Tue, 15 Jun 2010 12:41:23 -0700 (PDT)

Message-Id: <01CA74A3-D4DB-462A-A314-99C36C917D7B@gmail.com>

(All the mydomain.com's and 0.0.0.0 IP addresses were edited in by me, FYI.)

Could the Message-Id header be causing this? I thought this was only used for threading messages. I can post the full message headers if anyone wants, but I think these are the only differences. Any ideas?