Soloway, 28, reportedly made $300,000 in 2005 through his junk-mail operation by sending millions of unsolicited emails worldwide.
In a hurried hearing on Friday, Soloway appeared before the U.S. District Court in Seattle, where he is scheduled to go on trial in two weeks for a 40-count indictment that includes seven counts of aggravated identity theft - which carries a mandatory two-year prison sentence to be added to any other sentence - and 13 counts of money laundering.
In exchange for Soloway's guilty pleas, federal prosecutors dropped all but one of the spam-related charges contained within the indictment.
However, Soloway still faces up to 20 years in prison for the more serious charge of non-electronic mail fraud, which originates from unfulfilled guarantees surrounding email-marketing software.
The charge of electronic-mail fraud is punishable by up to five years in prison, and the misdemeanor tax charge carries a maximum sentence of one year.
Soloway and his business may be fined as much as $625,000.
Both sides of the case chose U.S. District Court Judge Marsha Penchman to decide Soloway's prison sentence and the amount of restitution he may be ordered to pay, based on the number of victims.
"We believe that there were extensive losses to thousands of victims," Assistant U.S. Attorney Kathryn Warma told The Seattle Times.
Soloway's attorney, Richard Troberman, said the government's dismissal of 37 counts, including all of the identity-theft cases, illustrates that the case "turned out to be very different from was originally charged."
"We feel that when it comes time for sentencing, we like our chances," he said.
Federal prosecutors said Soloway defrauded customers who paid him to send out high volumes of commercial e-mail or bought his software to send spam themselves, according to The Seattle Times. Customers reportedly could enlist Soloway to send emails to 20 million addresses for 15 days or sell them 80,000 email addresses for $495.
Early reports accused Soloway of sending penis-enhancement ads and pornography through his company's servers, but the accusation was found to be false after the federal government examined the severs operated by Newport Internet Marketing, Troberman said.
Warma said the government still believes Soloway is responsible for some of those acts and intends to present evidence to Pechman at Soloway's sentencing on June 20.
Soloway also is named in a default judgment of more than $10 million in Oklahoma and owes Microsoft as a result of another judgment.