How does that work with encrypted transport? Yahoo, Microsoft, Google and of course many others, all provide IMAP over TLS so sending email to them and receiving from them doesn't go in the clear.
They could still store metadata like sending IP address, timestamp etc.
Using foreign webmail via HTTPS would not be intercepted in any possible way. Unless they get support from the service provider in question. I think the service providers you mention will comply with law enforcement and give out your data. But those are isolated cases and mass data required by intelligence services is a different thing. National security letters are only for US Govt use and other governments have no access.
I also think legislation like this is years late now that about every protocol has encrypted variants and they don't get any meaningful results compared to the money spent.
(Sarcasm, but a non-zero fear that it's true.)
The problem is that even though you're trivially able to detect that TLS is not in use, the vast majority of mail providers won't act on that knowledge by refusing to send mail unencrypted (except maybe for some hosts explicitly whitelisted for that approach).
Why? Too many broken TLS setups, historically. Might be better now, I vaguely remember some push towards that from the big providers.
Alternatively, the government could also conduct a TLS certificate man-in-the-middle, which would work in most cases since almost no MTAs validate certificates outside of occasionally trying DANE (a spec for pinning certs over DNSSEC).