An affair developed between the pair and continued for three years until it ended in February 2014.
The relationship, which was hidden from the man's wife, identified as Mrs X, included a "significant number of liaisons at various locations around the country".
"It is the fallout of the relationship that saw the laying of charges in this court on 3 June, 2015," the decision said.
Kronfeld denied the harassment and intentionally accessing a computer without authority charges.
The harassment charge relates to April 5, 2015, when Kronfeld sent five messages to the man's wife using pseudonyms.
They followed many other emails, text messages and "malicious and fictitious phone calls" which were part of Kronfeld "orchestrating a premeditated, persistent, vile barrage and attack on Mrs X".
It was conceded at the hearing that Kronfeld had sent, or caused to be sent, a large quantity of emails and texts to Mrs X.
She had created at least 11 fictitious email addresses to bombard Mrs X.
The court heard the April 5 emails upset Mrs X.
The emails took aim at Mrs X's husband's fidelity in explicit fashion.
"She said that she was being taunted/played with and that Mrs Kronfeld was constantly trying to frighten her and wanted her to beg," the decision said.
"She was in a 'nightmare' and said that some of the messages were rude."
She found some of the communication "revolting and vile" and feared for her children.
Kronfeld accepted she had a long affair with the man and it ended in 2014 after her business partner said he was aware of the relationship and she had to "get it sorted".
Kronfeld said she sent the emails to irritate Mrs X and did not think of the effect they would have or intend to harm her physical or mental health.
Judge Crosbie did not accept Kronfeld's evidence during the hearing and thought it lacked credibility.
He found she was an evasive witness, who "lacked candour" and that she had been "attempting to cover her tracks in a relatively sophisticated manner".
"The sustained nature of the contact, the deception involved and the crude, vile and offensive content of many of the communications paints a more accurate picture of Mrs Kronfeld's intent," Judge Crosbie said.
Kronfeld was remanded on bail until her sentencing on April 28, which will be conducted via audiovisual link from the Auckland District Court.
Criminal harassment carries a maximum penalty of two years' jail.
