The Daily Mail has reported that the document contains allegations of racist, sexist and homophobic text messages and Mackay now looks to be out of the running to succeed Tony Pulis at Palace
The Welsh club have sent a report to the Football Association detailing a number of issues related to when former manager Mackay and Iain Moody, the Bluebirds' former head of recruitment, were at Cardiff
Moody resigned as Palace sporting director on Thursday as the FA revealed it was investigating allegations of misconduct during his time at Cardiff
Press Association Sport understands the dossier was filed more than a week ago and its timing had nothing to do with Mackay being strongly linked with the Palace job
Cardiff owner Vincent Tan sacked Mackay in December 2013, just a few months after the Scot had ended the Bluebirds' 51-year wait for top-flight football
Two months earlier Moody had been placed on gardening leave before being subsequently dismissed.
Mackay launched a £7.5million legal claim against Tan for compensation and wrongful dismissal after his sacking but suddenly dropped the claim in May and issued an apology to the Malaysian businessman.
Cardiff have declined to make any public comment on the dossier and the matter is now in the hands of the football authorities.
An FA spokesman said: "The FA can confirm it is currently investigating this matter."
Kick It Out chairman Lord Herman Ouseley believes it is vital that, in cases where wrongdoing is proven, those involved in the game should not be able to excuse themselves by pointing to the fact that their comments or actions were not intended for public consumption
Ouseley said football continued to be "tainted" with discrimination and that "the football establishment knows and condones it".
He also said that when private exchanges become public knowledge that "the individuals concerned must accept the full consequences of their actions".
He added: "What you see at face value is not always reflective of the attitudes which are actually held deep down
It's easy to present yourself as being reasonable and fair, and to behave in ways that hide prejudice and bias.
"However, any in-depth forensic examination or analysis of hidden views would be likely to reveal otherwise
That is what makes it difficult to eliminate and eradicate such prejudices, bigotry and even hatred.
"The governing bodies and the clubs must denounce such attitudes prevalent in the game, and take the appropriate action."
Tim Hartley, chairman of the Cardiff City Supporters' Trust, said: "The Trust condemns without reservation any racist, homophobic and sexist comments whenever and wherever they are made
"We are not privileged to the details of the dossier concerning alleged comments made by our former manager Malky Mackay.
"The Football Association has been sent documents by the club and we await the outcome of any investigation
It would be wrong of us to speculate further at this stage without any detailed information being made available."
Moody followed Mackay to Cardiff from Watford following the Scot's appointment in the summer of 2011
Moody had worked as a football writer and agent before joining the Watford press office
He later became head of football operations at the Hertfordshire club.
Earlier this week Palace were fined by the Premier League for their part in the 'spygate' saga involving Cardiff last April
The Premier League determined that Palace had breached its 'good faith' rule - B16 - by obtaining information about Cardiff's team ahead of their 3-0 win when the two clubs were relegation rivals.
The Welsh club had complained to the Premier League that Moody had contacted Cardiff employees for information in the build-up to the game, an accusation Moody denied.
Source : PA
Source: PA