You're speaking of the third party doctrine, which isn't strictly law, more a collection of precedent, and has never been ruled to actually apply to retained email. If you read the 6th Circuit's decision, you'll see a number of statutes that directly contradict the 180-day provision, and which give an additional legal basis for the protection of emails beyond just an interpretation of the 4th as overriding the law and striking it as unconstitutional.

The IRS trying this would be a great benefit to us all, short of congress getting their act together and revising the ECPA, and the EFF would love nothing more than to take up the case. Considering that it has already been confirmed that Google is requiring warrants and Microsoft has taken such a strong stand while releasing their latest transparency report, it seems like we also have at least two corporate sponsors (and, actually, this is a bad position for most companies to be in, as many email providers can't easily figure out which Appeals Court their customer falls under. This exposes them to risk if they disclose email content without a warrant, which is another motivation to take the conservative approach and ask a court to decide if they have to disclose emails with only a subpoena).