In Garnett v. Commonwealth, No. 1573-15-2 (Virginia Ct. App., Dec. 20, 2016), the Virginia Court of Appeals reversed the trial court’s admission of text messages against the defendant because the prosecution had not carried its burden of proving that the defendant had owned the cell phone in question. Citing cases from Mississippi, Texas, and Illinois, the court described several non-exhaustive ways by which to prove that someone sent an electronic communication: (1) "the purported sender admits authorship;" (2) "the purported sender is seen composing the communication;" (3) "business records of an internet service provider or cell phone company show that the communication originated from the purported sender's personal computer or cell phone under circumstances in which it is reasonable to believe that only the purported sender would have access to the computer or cell phone;" (4) "the communication contains information that only the purported sender could be expected to know;" or (5) "the purported sender responds to an exchange in such a way as to indicate circumstantially that he was in fact the author of the communication."
In Garnett, the prosecutor offered evidence only that the cell phone was found in the car with the defendant, who was the only occupant. The common-sense inference may be that the defendant owned the phone, but the court required more and ruled that the trial court had abused its discretion.
While Garnett was a criminal case, the test for authenticity of electronic communications probably does not differ much in civil cases in most jurisdictions. Likewise, while Garnett dealt with text messages on cell phones, the same principles apply to any number of electronic communications on various devices.
The upshot? Make it a habit to issue a subpoena to the service provider for authenticating documents (though you should expect pushback from those companies). Absent that, you might need to secure testimony from someone who communicated with the author in question at the cell phone number, email address, etc. in question. Or perhaps you will be lucky and find a message that speaks for itself in that the purported author reveals peculiar personal knowledge or otherwise effectively admits his or her identity.