Normally I would open Mail.app, click Message → Encoding → [One of the Cyrillic encodings]. But since the upgrade I don't see text encoding option in Message menu.
How to change to use one of the Cyrillic encodings?
It seems you're not the only one with this problem.
I found a KB article on Apple's website that clearly specifies that Message > Text Encoding should be available in the Mavericks version of Mail:
It says "Mavericks" in the title and is current as of Nov 6, 2013.
I've filed a bug report letting the OS X team know.
Have the same problem. Tried the thread from Alex comments (here), seems can solve my problem. But Apple should bring back the Text Encoding option.
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