Horner, who denied the accusations, was cleared on Wednesday after a lengthy investigation carried out by an independent lawyer for the championship-winning team's Austrian parent company, Red Bull GmbH. Red Bull has said the investigation report will remain confidential.
Horner told Sky Sports on Thursday he was focusing on Saturday's season-opening race at Sakhir but his hopes of swiftly moving on were rocked by the email sent around the paddock.
"Following Red Bull’s recent investigation and statements you will be interested to see the materials attached," the email, sent from an anonymous address and headed 'Christian Horner investigation evidence', said over a Google Drive link.
Some 150 recipients included the president of the governing FIA Mohammed Ben Sulayem, Formula 1 chief executive Stefano Domenicali, commercial rights holders Liberty Media, fellow team bosses and members of the media.
The email triggered renewed speculation, in the media and on social networks, about Horner’s future at the sport's dominant team.
An FIA spokesperson said on Friday that the governing body currently considered it an internal matter but was keeping a close eye on the situation.
In a statement on Thursday night, Horner stood by his denial of the allegations.
"I won’t comment on anonymous speculation," said the 50-year-old, who is married to former Spice Girls singer Geri Halliwell.
"But to reiterate I have always denied the allegations. I respected the integrity of the independent investigation and fully cooperated with it every step of the way.
"It was a thorough and fair investigation conducted by an independent specialist barrister and it has concluded dismissing the complaint made. I remain fully focussed on the start of the season."
Mercedes and McLaren team bosses Toto Wolff and Zak Brown had earlier called on Formula 1's governing body to push Red Bull for greater transparency regarding the investigation.
Horner is the sport’s longest-serving team boss having brought Red Bull, whose current drivers are Dutch triple world champion Max Verstappen and Mexican Sergio Perez, into Formula One in 2005.
Last year, he led the Milton Keynes-based squad to its sixth constructors’ title with 21 wins from 22 races in the most dominant campaign in the sport’s history. Verstappen, winner of 19 races, clinched his third successive crown.
Drivers have a final practice session on Friday before qualifying for Saturday’s race.
