Discovery of the day: quick and easy Apple Mail connection tunneling

I'm on a wireless network which apparently has a firewall which is MITMing port 993 connections (IMAP-over-SSL); my mail client reported a certificate error (the presented certificate was from “FortiGate”). Now, I trust this network's provider, but that doesn't mean I'm going to give them my personal e-mail, much less the password for it; so I went looking for a solution. It turns out that Apple Mail supports SOCKS proxies, and if you have SSH access to another system it's trivial to set up; ssh -D somelocalport somehost, then go into Network Preferences → Advanced → Proxies, and enter localhost:somelocalport as the proxy, and you're done!

It's not clear to me, though, once I set this up how much of my traffic goes over the proxy — the setting is not specific to mail or Apple Mail. This might be testable by shutting down the proxy and seeing what fails.