By the Grace of G-d Rosh Chodesh Elul, 5746 Brooklyn, N. Y.
Rabbi Dr. Dov Mandelbaum
55 Bonita Vista Road
Mount Vernon, N. Y. 10552
Greeting and Blessing:
I duly received from Rabbi Groner the letters with reference to me, more precisely, to my failure to respond to your correspondence.
Let me say at once that your complaints are completely justified. I can only ask your indulgence to judge me “in the scale of merit.”1 I assure you that my failure to respond does not reflect anything in the nature of “cold shouldering,” as you put it ח"ו. As for an explanation of the absence of written responses on my part - I plead “guilty without explanation,” since an explanation would have to be rather lengthy and beyond the scope of this letter. I must therefore rely on the כף זכות, that you will believe me when I say that the cause has absolutely nothing to do with you.
The reason I am writing to you now is not hard to find, since we have entered the month of Elul, when it is time to make amends for past failures. And although, obviously, these few lines cannot quantitatively suffice for all that has been omitted, they, hopefully, can suffice in terms of quality and real content.
As you know, the month of Elul is explained by the well-known allegory of the Alter Rebbe as the time when “The King is in the field.” This is the time when the King is pleased to be greeted by the workers in the field, and is particularly gracious to accept their petitions (Likutei Torah, Re'eh 32, col. 2). All of us, together and individually, are of course, included in this extraordinary and auspicious opportunity.
May it indeed be so with you and your wife, to enjoy many, many good and pleasant years together. And “good and pleasant” in your case includes the fullest utilization of the capacities that Hashem has bestowed on you for the benefit of the "Prat" and "Klal."
With prayerful wishes for a Kesivo vaChasimo Tovo, for a good and sweet year, and
With esteem and blessing,
M. Schneerson
Notes and sources
Rabbi Mandelbaum was a respected rabbi that was close to Lubavitch and defended the Rebbe's stance in regards to "Who is a Jew" in many newspapers. He wrote a letter to the Rebbe, but for some reason, did not receive a reply. He then penned a letter to one of the Rebbe’s aides complaining why his letter was ignored. This letter is the Rebbe’s response to the rabbi.
See follow-up letter dated "In the days of Selichoth, 5746".
1) Avos 1:6.
Source & Link: אס"ל ח"ד, pg. 197: https://drive.google.com/file/d/1eb06rAYurdTMQcb6XEX13PMaKKIf9BI-/view?usp=drivesdk
Alternative Source:
https://www.chabad.org/therebbe/letters/default_cdo/aid/695379/jewish/The-Rebbes-Apology.htm