You're confusing the conversation here (collective "you"). To be clear, if you use a third party mail service, you are bound by their terms of service, and the laws of the country in which they operate. Depending on where the service is provided and other jurisdictional quibbling of course.
Your argument seems to be more along the wiretapping vein - what they do without asking. That is a completely separate argument, and unrelated. The difference in the Warshak decision was that compelling a provider required a warrant. If the provider fulfills a request without objection, no compelling took place and no warrant is required. I can't tell why the ACLU would expect otherwise.
If I send mail outside of my country, I would not have any reason to expect those to remain protected by any statute or term of service, so that's effectively a public e-mail.
If I an in the United States, and my provider says they will comply with law enforcement requests, it doesn't matter what any law says. My provider will probably hand over information without a warrant. Any agency of the local or federal law enforcement can just phone, write, or ask in person for whatever they want to know. That's a public e-mail, no matter who expects it to be private.
Now, what if your provider says no, or has said no enough in the past that law enforcement doesn't feel like asking politely? This is apparently a corner case. And it is the only one to which your argument is applicable. And, your expectation of privacy is just as unrelated, because it is your provider's decision to fight the request on your behalf. If the agency gets a court order or warrant, it is constitutional and your expectation of privacy vanishes.
The IRS is saying that if they suspect tax evasion, they will ask your provider for e-mail records. They will use strong-arm tactics like the Electronic Communication Protection Act to encourage hesitant providers. They will do anything and everything they can to get your mail, legally, without a warrant. This is not surprising, nor is it a Fourth Amendment violation. Fourth Amendment protections are from illegal searches, not legal ones. If they ask for records and your provider provides them, that's a legal search. If they compel the provider without a warrant, that is currently neither constitutional nor unconstitutional - it is undecided outside of the Sixth Circuit. Opine and argue if you like, but it is undecided. And outside of the Sixth Circuit, an appeal could be decided in favor of the government.
But here's the catch. No one who has mail with a third party provider is going to have standing to question the legality of a search when the provider did not object. You may have a complaint against your provider, but that will change neither the policy of law enforcement nor the law.
Argue about principles all you want, but unless you specifically choose an e-mail provider which guarantees they will reject any requests other than court-ordered, you have nothing to complain about except your poor choice in provider. If you send mails to a provider without those same terms, your sent mails could become private. Your recipients could voluntarily turn over your mails without a warrant, for that matter.
The only question is, does such a provider exist? It would be awfully expensive to have a fleet of lawyers to take on fighting every request until reaching the Supreme Court.
... the IRS hasn't told the public whether it is following Warshak everywhere in the country, or only within the Sixth Circuit.â
This only matters if the provider objects, and the IRS does not feel like getting a warrant, and the provider is outside the Sixth Circuit. If there were a known case of this, I think the ACLU would be fighting for that person instead of tilting at windmills here.