I have read Mosman's decision (the first two paragraphs of the PDF) and skimmed through the background info, (the bulk of the rest of the document) and I have to say that I agree with him.
First, he's not saying that a warrant is not required for law enforcement to search your email. All he's saying is that they are not *required* to actually tell the account holder that the email has been searched. They still have to present the warrant to the email host.
This is exactly the state of affairs when it comes to physical searches of property stored at a 3rd party location. If I have stuff at a storage locker somewhere, police will take their warrant to the management office of the storage facility and say "Let us into the locker." Legally speaking, that's all they're required to do.
Now, in either case, I would expect to be notified by the host or storage facility that they complied with the search warrant. That's just good business.
Where the original case makes me uneasy isn't that the warrant was only given to the ISP. It's that the warrant included a supplemental gag order preventing them from telling anyone, INCLUDING THE ACCOUNT HOLDER, that they were complying with the search warrant.
IMHO, and I'm by no means a legal professional, they chose the wrong grounds upon which to try to appeal the decision. Rather than appeal on the grounds that they weren't notified of the search, they should have appealed on the grounds that the gag order was unjustified. I don't know that it would necessarily have gotten them anywhere, either, but it'd at least be a stance I can agree with.
Yet another interesting point is made in Mosman's discussion. In a normal, physical, search and seizure, they're required to leave a list of things they take. That way, you know what should be missing and can reclaim your property after the trial (if you're in a position to do so).
He argued that because nothing was actually *taken* -- the police simply made copies of their mail folders -- there was no need to leave a receipt, which would have served as a different means of alerting the account holder that their account had been searched.
This could have some potentially interesting consequences in physical searches as well. What if the police execute a search warrant for documents and rather than physically taking the papers to the police station, they instead set up a few high-speed duplex scanners and just scanned in the documents on the premesis? If that's all they do, they're not "depriving you of property" so there's no reason to leave a receipt.