In his online post, Mr Mann said the document had arrived “beautifully printed out by recorded delivery”.
The letter’s content, he said, attacked his efforts to tackle antisemitism on the left.
He said he had contacted the GP, and the pensioner had told him he did not use a computer and had not been to the post office.
Mr Mann continued: “But he was very happy to chat and we did for a long time. He is a very gentle man, polite in a way that symbolises his generation. He has views, clear and precise and we agreed on some things but not all. Like most civilised people.
“But he never once attacked nor abused me. He was mortified to hear what had been written in his name. He would never dream of writing to someone telling them how much he hated them and he was interested to be sent copies of what I had said and written on antisemitism.”
Mr Mann said the GP had put him in touch with “the man who actually wrote the letter”, who had shouted at the MP before putting the phone down.
The activist, the MP said, “should have the courage to say things yourself and not masquerade as a 90 year old doctor”.
But today, Mr Greenstein published a blog post in which he said he had spoken to both the doctor and the activist, and claimed the pair had worked together to write to Mr Mann.
After speaking to the men, “it became clear to me very quickly that the original letter was not a forgery”.
Mr Greenstein published a second, hand-written, letter from the doctor.
“John Mann’s allegations of a forgery are classic Zionist tactics. Attack the messenger and avoid the message,” Mr Greenstein wrote.
Mr Mann has been a leading campaigner against antisemitism and in the past year has challenged members of his own party about their comments and actions, including Ken Livingstone, the former Mayor of London who is currently suspended from Labour for suggesting Hitler was a Zionist.
