The alleged bomb threat cited by Belarus as the reason why it diverted a plane to Minsk before arresting a journalist on board was sent after the plane was redirected, the email provider said.
Proton Technologies, which is headquartered in Geneva and says it is the world's largest secure email provider, said the sent time confirmed it came after the Ryanair passenger plane was diverted to Belarus.
The Athens-to-Vilnius flight carrying a wanted opposition activist was forced to land in Minsk on Sunday over a supposed bomb scare.
"Due to our encryption, we can't access/verify the message contents," ProtonMail said on Twitter.
"However, we can see the sent time and can confirm it was after the plane was redirected.
"Based on the screenshots of the leaked email that were posted online, it was leaked by somebody with access to the receiving mail server in Belarus. The receiving mail server also has a copy of the email."
It has also emerged Belarusian air traffic control refused a request by the Ryanair pilot to contact the airline after being told of the alleged bomb threat, leaving him with no alternative but to land in Minsk, Ryanair said in a letter seen by Reuters.
In a letter to the Belarus transport ministry dated 26 May, Ryanair Chief Executive Micheal O'Leary described previous correspondence from Belarusian officials as "false and inaccurate" and said the plane had been "unlawfully diverted under false pretences".
"The pilot in command was left with no alternative but to divert to Minsk, when he was advised by Minsk ATC (Air Traffic Control) that there was a credible bomb threat to the aircraft, yet Minsk ATC refused to contact Ryanair, falsely claimed tha tRyanair Ops would not answer the phone," the letter said.
The pilot repeatedly requested information about the alleged bomb threat before ultimately agreeing to land in Minsk, according to a transcript released on Tuesday by authorities in Belarus.
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Belarusian President Alexander Lukashenko sparked outrage by sending a fighter jet to intercept the Ryanair flight. After landing in Minsk, dissident journalist Roman Protasevich and his girlfriend were arrested.
The Belarusian authorities claimed that Minsk had received a threatening email from a ProtonMail address purportedly from the Palestinian Islamist group Hamas claiming that a bomb was on board flight FR4978.
At around 0930 GMT on Sunday, the Boeing 737-800 plane entered Belarusian airspace and was contacted by Belarusian air traffic control.
According to the transcript of exchanges between the control tower and the plane, they informed the pilot of the threat at 0930 GMT and a minute later recommended landing in Minsk.
At 1015 GMT, the flight landed in Minsk National Airport.
According to Belarus, the email said: "We, Hamas soldiers, demand that Israel cease fire in the Gaza Strip. We demand that the European Union abandon its support for Israel in this war.
"A bomb has been planted onto this aircraft. If you don't meet our demands the bomb will explode on May 23 over Vilnius. Allahu Akbar."
Air France, meanwhile, cancelled another flight from Paris to Moscow today after Russia rejected a flight plan that would have skipped Belarusian airspace, the airline said.
A flight was scrapped on Wednesday for similar reasons as the EU has urged airlines to avoid Belarusian airspace.
Russia's aviation authority today notified airlines that changes to routes from Europe to Russia may result in longer clearance times due to an increase in requests.
