I have a Mac Pro at home running Snow Leopard 10.6.8 and a Mac mini at work running Mavericks 10.9.1. I basically live and die via e-mail. Mail 4.6 from 10.6.8 is the single best e-mail program I've ever used. It's perfect for me and my particular workflow. I hate Mail 7.1 on Mavericks 10.9.1. While it tries hard to be like the old Mail it is dog slow and they took away valuable features (like click-drag to select multiple messages - WTF?!? ) that I depend on every day. I would love to be able to run the old Mail on my work machine (it would also give me an incentive to upgrade the home machine, if it worked). Obviously there'd be a ton of Frameworks/Private Frameworks issues (Mail 4.6 uses the Carbon, iLifeMediaBrowser, InstantMessage, ISSupport and Message Frameworks that don't exist in 10.9.1, to name just a few examples) but I've had past experience playing around with installing old Frameworks into ~/Library/Frameworks and doing things with DYLD_LIBRARY_PATH et al. and other fun sandboxing type trickery to get older binaries to run on occasion. Anyone else ever considered trying it?