A letter such as this doesn’t add up to anything as far as evidence that the Holocaust didn’t happen.
There is, of course, a mountain of evidence in the form of photographs, records, and eyewitness accounts (and more) that the Nazis planned to kill all the Jews of Europe and succeeded in killing ~6 million of them. Some of the most damning are the recordings of speeches by Heinrich Himmler, including the following:
I ask of you that that which I say to you in this circle be really only heard and not ever discussed. We were faced with the question: what about the women and children? – I decided to find a clear solution to this problem too. I did not consider myself justified to exterminate the men – in other words, to kill them or have them killed and allow the avengers of our sons and grandsons in the form of their children to grow up. The difficult decision had to be made to have this people disappear from the earth. For the organisation which had to execute this task, it was the most difficult which we had ever had. […] I felt obliged to you, as the most superior dignitary, as the most superior dignitary of the party, this political order, this political instrument of the Führer, to also speak about this question quite openly and to say how it has been. The Jewish question in the countries that we occupy will be solved by the end of this year. Only remainders of odd Jews that managed to find hiding places will be left over.
Also:
I am now referring to the evacuation of the Jews, the extermination of the Jewish people. It’s one of those things that is easily said: ‘The Jewish people are being exterminated’, says every party member, ‘this is very obvious, it’s in our program, elimination of the Jews, extermination, we’re doing it, hah, a small matter.’ And then they turn up, the upstanding 80 million Germans, and each one has his decent Jew. They say the others are all swines, but this particular one is a splendid Jew. But none has observed it, endured it. Most of you here know what it means when 100 corpses lie next to each other, when there are 500 or when there are 1,000. To have endured this and at the same time to have remained a decent person — with exceptions due to human weaknesses — has made us tough, and is a glorious chapter that has not and will not be spoken of. Because we know how difficult it would be for us if we still had Jews as secret saboteurs, agitators and rabble-rousers in every city, what with the bombings, with the burden and with the hardships of the war. If the Jews were still part of the German nation, we would most likely arrive now at the state we were at in 1916 and 17 […]
If you insist on continuing your interaction with the denier, see what he has to say about Himmler’s own words.