She was never able to produce the email, and a forensic computer expert found no trace of it.
McDonald said she had also called in a computer expert who told her it was likely to have been deleted by a virus.
A document examiner from New Zealand police said he could not be sure, but believed the signature on the document to be false.
The tenant eventually bought the property through the owner's lawyer, and McDonald lodged a caveat to enforce her agency agreement and claim commission.
The tribunal found her guilty on all four charges and said her conduct, using dishonest and forgery to assert her entitlement to commission, was "disgraceful".
A penalty has not yet been set.
Meanwhile, in other READT decisions, agents Astrid and Karen Clark were censured and fined $3000 each.
The tribunal had previously found both guilty of unsatisfactory conduct by not disclosing in writing that Karen Clark stood to benefit from a sale, but also that the breaches were not deliberate.
In another case, complainants appealed against a decision not to take action against three agents, Patrick Regan, Mary Curnow and Lisa Hopewell.
The tribunal found Mr Regan guilty of unsatisfactory conduct, but dismissed the appeal in relation to the other agents. A penalty for Mr Regan has not been set.
Complainants also appealed against a decision to impose no penalty on Kapiti agent Della Randall for not seeking further information about weathertightness issues. The appeal was dismissed.