Jaynes, 30, his sister, Jessica DeGroot, 28, and a third defendant, Richard Rutkowski, all from North Carolina, were charged by Virgina state prosecutors with violating the state's tough anti-spamming laws, which came into force in July last year.
The legislation was the first in the US to introduce custodial sentences for the worst offenders, and applies when the spammer:
Jaynes, alias Gaven Stubberfield, was charged with sending out over 100,000 spam messages in July and August last year, reports Reuters.
"This was just a case of fraud," said prosecutor Gene Fishel, according to the Associated Press. "This is a snake oil salesman in a new format."
While living in North Carolina, the three defendants found themselves charged in Virginia because they had used an AOL server, based in that state, to target the ISP's customers.
On Wednesday the jury acquitted Rutkowski but found Jaynes and his sister guilty of violating the new law. It has recommended that Jaynes serve nine years in prison, and that DeGroot be fined $7,500.
Trial Judge Thomas Horne has still to confirm the sentences and, according to the Associated Press, is yet to rule on two motions to dismiss the cases against Rutkowski and De Groot.
State Attorney General Jerry Kilgore called the verdict a victory "for Virginians and all Americans".
Jaynes' lawyer, David Oblon, expressed shock at the recommendation, telling the AP, "Nine years is absolutely outrageous when you look at what we do to people convicted of crimes like robbery and rape."