The court heard the emails upset the businessman's wife.
"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.''
Kronfeld accepted she had a long affair with the man and it ended in February 2014.
She sent the emails to irritate the man's wife and did not think of the effect they would have and did not intend to harm her physical or mental health.
Judge Crosbie did not accept Kronfeld's evidence and thought it lacked credibility in many respects.
The charge was proved beyond reasonable doubt, he said.
Kronfeld will be sentenced via AVL on April 28.