That number is the highest monthly total since Postini began tracking phishing in January. July's total breaks June's record of 16.7 million phishing emails, the company said.
While phishing attacks increased, the number of emails containing viruses decreased in July by 20 percent compared to June, Postini said.
The amount of spam remained stable at 88 percent of the total number of emails sent. The company processed more than 14 billion emails last month.
Directory harvest attacks decreased 8 percent from June.
Gartner researchers have estimated that online debit card fraud, perpetrated via phishing and keystroke logging attacks, has resulted in $2.75 billion in losses in the past year.