The pan-European exchange announced last month it would not make a counterbid for Spain's BME, leaving a clear run for Swiss Exchange SIX to complete its takeover of the Madrid bourse.
Boujnah said in an interview the price and terms were not right for Euronext, calculating the Zurich exchange operator is paying almost a 40% premium for an exchange that has suffered declining earnings in recent years.
Zurich has also committed to making no synergies for a decade, which would cap its return on equity, he said.
"This is bound to raise some eyebrows," Boujnah said.
SIX had no comment on Monday, but noted its chief executive Jos Dijsselhof said last month the price reflected the value of BME as the Swiss exchange creates Europe's third biggest market infrastructure group.
Since 2018, Euronext has added Dublin and Oslo exchanges to a stable that also includes operations in Brussels, Amsterdam, Lisbon, Paris and London.
Acquiring another bourse is still an option, Boujnah said.
The London Stock Exchange (LSE) is buying data and analytics company Refinitiv for $27 billion, leading to speculation that it might have to offload its Italian operations to get the green light from European Union competition regulators.
While the LSE has made it clear it has no intention to analyse a potential sale of Borsa Italiana, fallout from the pandemic could change this, Boujnah said.
"The company is not for sale but after the crisis and the dust settles, who knows? What we have done is register an interest to the LSE, and we will see," he said.
LSE CEO David Schwimmer said in February Borsa Italiana remained an integral part of the business.
Euronext is also looking to buy into data, analytics and post-trade services.
"We are looking again at valuations at a moment when trading is being redefined and can have an impact on the outlook for the multiples that we are willing to pay," Boujnah said.
The pandemic has triggered record trading volumes for exchanges, coupled with bouts of extreme volatility, at a time when nearly all staff, including Boujnah, are working from home due to national lockdowns.
"This crisis is the ultimate stress test on exchange operators and we have passed," Boujnah said. "Everyone understood that closing the market would have created chaos and plenty of unwanted consequences."
Euronext share trading transactions in March were up 92% on February, and 197% on March 2019.
With 40% of Euronext's revenues driven by trading volumes, Boujnah said this would mean "material progress" for the bourse's top-line.
By Huw Jones