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We have, we wrote Thunderbird, and cos its open source you can improve it.
(By 'we' i mean the open source community, in which i include myself, though I havent done any of the work yet!)
[g]
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the heart of Thunderbird is an open source development process driven by thousands of passionate, experienced developers and security experts spread all over the world. Our openness and active community of experts helps to ensure our products are more secure and quickly updated, while also enabling us to take advantage of the best third party security scanning and evaluation tools to further bolster overall security.