<div class="markdown">
<p dir="auto">I have attempted to define custom mailbox icons in Mailboxes.plist, but <strong>only</strong> the Mail.app icons work. Here is the delta from my Mailboxes.plist file:</p>

<pre><code>    {    deltaMailboxes = (
    {    previousUUID = "";
        uuid = "A42F93E8-612A-4838-AAC6-733D1D766B5E";
    },
    {    imagePath = "/Applications/Mail.app/Contents/Resources/InMailbox.tiff";
        previousUUID = "A42F93E8-612A-4838-AAC6-733D1D766B5E";
        uuid = "INBOX";
    },
    {    imagePath = "/Applications/Mail.app/Contents/Resources/DraftsMailbox.tiff";
        previousUUID = "INBOX";
        uuid = "DRAFTS";
    },
    {    imagePath = "/Applications/Mail.app/Contents/Resources/SentMailbox.tiff";
        previousUUID = "DRAFTS";
        uuid = "SENT";
    },
    {    imagePath = "~/Dropbox/Icons/mailbag.png";
        previousUUID = "SENT";
        uuid = "ALL_MESSAGES";
    },
    {    previousUUID = "";
        uuid = "ALL_BODY_PARTS";
    },
    {    imagePath = "/Applications/Mail.app/Contents/Resources/ArchiveMailbox.tiff";
        previousUUID = "ALL_MESSAGES";
        uuid = "ARCHIVE";
    },
    {    imagePath = "/Applications/Mail.app/Contents/Resources/JunkMailbox.tiff";
        previousUUID = "ARCHIVE";
        uuid = "JUNK";
    },
    {    previousUUID = "JUNK";
        uuid = "TRASH";
    },
    {    previousUUID = "TRASH";
        uuid = "TAGS";
    },
    {    previousUUID = "TAGS";
        uuid = "MAILING_LISTS";
    },
    {    previousUUID = "MAILING_LISTS";
        uuid = "120891A9-394F-4B0E-AF9E-79B2C2B9700A";
    },
    {    previousUUID = "CD87BA82-58F0-417D-AD74-F78A0EEC1BAD";
        uuid = "1445D2F2-31B1-4338-9A60-81E08A97064B";
    },
    {    previousUUID = "1445D2F2-31B1-4338-9A60-81E08A97064B";
        uuid = "FLAGGED";
    },
    {    previousUUID = "FLAGGED";
        uuid = "020C06F6-BA46-4831-ABF0-B8B5A84DD434";
    },
    {    previousUUID = "020C06F6-BA46-4831-ABF0-B8B5A84DD434";
        uuid = "E65A53B0-7647-4A57-97DB-28A564AAB208";
    },
    {    previousUUID = "E65A53B0-7647-4A57-97DB-28A564AAB208";
        uuid = "16C65DF0-DD76-4141-B24B-733EAA25E4C4";
    },
    {    previousUUID = "16C65DF0-DD76-4141-B24B-733EAA25E4C4";
        uuid = "9BF3B4D3-EE1D-443D-8674-D9928DD25D49";
    },
    {    previousUUID = "9BF3B4D3-EE1D-443D-8674-D9928DD25D49";
        uuid = "PERSONAL_INBOX";
    },
    {    previousUUID = "PERSONAL_INBOX";
        uuid = "B6C2807D-87C8-40BA-8713-6CB65F1C5162";
    },
    {    previousUUID = "B6C2807D-87C8-40BA-8713-6CB65F1C5162";
        uuid = "53EF7960-D837-4A87-9E7F-C91F9323E4EB";
    },
    {    previousUUID = "53EF7960-D837-4A87-9E7F-C91F9323E4EB";
        uuid = "9E84B53A-6D10-4D0C-96E9-C6322DCFD691";
    },
    {    imagePath = "~/Dropbox/Icons/Lupkin.png";
        previousUUID = "";
        uuid = "imap://me@them.com";
    },
    {    imagePath = "~/Dropbox/Icons/YellowGlasses.png";
        previousUUID = "imap://me@them.com";
        uuid = "iimap://me@them.com";
    },
    {    imagePath = "~/Dropbox/Icons/gmail.tiff";
        previousUUID = "imap://me@them.com";
        uuid = "imap://me@them.com";
    },
    {    imagePath = "~/Dropbox/Icons/trash_mail.png";
        previousUUID = "imap://me@them.com";
        uuid = "imap://me@them.com";
    },
    {    imagePath = "~/Dropbox/Icons/glasses-scale-border-logo.png";
        previousUUID = "imap://me@them.com ”;
        uuid = "imap://thenerdlawyer@imap.gmail.com";
    },
    {    previousUUID = "imap://me@them.com";
        uuid = "imap://me@them.com";
    },
);
</code></pre>

<p dir="auto">And here is what the mailbox outline looks like:</p>

<p dir="auto"><a href="https://dl.dropboxusercontent.com/u/24123516/Screen%20Shot%202016-03-18%20at%202.26.43%20PM.png">https://dl.dropboxusercontent.com/u/24123516/Screen%20Shot%202016-03-18%20at%202.26.43%20PM.png</a></p>

</div>