How about including an element in the email which points to a server-side script (e.g. PHP)? The script would generate the captcha image on-the-fly when requested.

----- Original Message ----
From: Christian Metzler
Sent: Wednesday, August 22, 2007 12:02:24 PM
Subject: Graphics Library for Thunderbird/XPCOM

In my extension I want to create Captcha images on the fly and send it by
mail. Is there a possibility to use a graphics libraryto create jpegs or
gifs from within Thunderbird?

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