Q: Does anyone know of a maintained managesieve client?

4 Years and nothing new?  I know OS X server even supports sieve with their IMAP and postfix mail implementatio. Sieve integration (visual instead of scripting) would fit very naturally into Mac Mail. They could ignore iOS and require rules to be setup on OS X mail. Sell more macs!

mac mail is the best mail client that exists to date. Very complete. With this exception.

This is the only one I've found that works:

https://github.com/philpennock/sieve-connect

I'm the author of that, so thank you.   (If anyone can figure out problems with MacOS/GSSAPI/Perl, please let me know).

If you use Homebrew, then this tap might be useful: https://github.com/philpennock/homebrew-protocols

(I updated that yesterday with the new release of `sieve-connect`)

For other clients which work, and there are quite a few, http://sieve.info/ is a useful index/resource.