The big condition: the destination country (ie: where the defendant is located) can’t object. Many countries do object, so if you need to serve in China, Germany, Mexico, or Switzerland (just to name a few), sorry. You can’t. It isn’t valid in those countries, regardless of Water Splash.
And perhaps even more critical: the forum court’s rules have to allow it. If the case is being heard in a jurisdiction that views mail service as invalid,* the Hague Service Convention doesn’t magically make it okay. There’s no fancy spell that Hermione Granger can conjure up to confer validity on mail service. Professor Dumbledore cannot simply wave his wand and make it okay. Harry Potter can’t just send Hedwig instead of hiring a process server. (I’ll stop.)
So the Supremes sent the Water Splash case back down to Texas, where, as luck would have it, the state courts don’t allow mail in the first place. After all that fighting over Article 10(a), the parties missed the far simpler argument.
Regardless, mail is a bad idea except in the rarest of cases anyway.
* My personal take: mail isn’t a means reasonably calculated to put a defendant on notice of the claim against him/her/it (see Mullane v. Central Hanover Bank & Trust Co., 339 U.S. 306 (1950)). Mail is almost as horrible a legal fiction as publication, when it comes down to it. Don’t get me wrong– I still think the U.S. Postal Service is a pretty effective outfit, despite how much it is maligned by the very people who govern it (I’m looking at you, Congress). But even though the Post Office might deliver a letter, that doesn’t reasonably ensure that the right person receives the envelope. If you have to mail it, I suggest a belt & suspenders approach: serve by mail, in conjunction with email and Facebook.