The three-judge panel affirmed the conviction of David Kernell, 24, for obstructing an investigation as a University of Tennessee student before agents seized his computer at his apartment in Knoxville.
Kernell's attorney, Wade Davies, contended at trial that Kernell had no criminal intent and that guessing his way into the email account when the then-Alaska governor was John McCain's running mate on the Republican presidential ticket was a prank.
Kernell was released in November after serving less than 11 months and Davies said he is doing well.
Davies said he will seek a review by the full U.S. 6th Circuit Court of Appeals.
Davies in an email statement Monday said his client was convicted of a misdemeanor for accessing email but also of "obstructing an investigation under a new statute that does not require there even to be an investigation pending."
Davies said the opinion "confirms that David Kernell did not have knowledge of an official investigation at the time of the alleged obstruction. We challenged the statute as unconstitutionally vague because it criminalizes conduct when someone only contemplates that there might be an investigation in the future."
Davies said that if necessary he will petition the U.S. Supreme Court to decide the constitutionality of "being convicted of a felony for obstructing an investigation without knowledge of the investigation."
The opinion Monday said the government presented sufficient evidence to support a conviction, showing that Kernell knew about the FBI investigation based on postings by "anonymous commentators" that they had reported him to the FBI and Kernell's posting himself that referred to the possibility of an investigation.
The jury that convicted Kernell in 2010 acquitted him of wire fraud and deadlocked on an identity theft charge.
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