The Orthodox Russian community was centred on the Andreevski Sikt / Sekte - basically a monastery but technically not eligible for that title. There was an Ottoman post office on Athos and from the 1890's a ROPIT post office.
Mail from Russia to Athos seems always to have been routed via Odessa and this fact is explicit on nearly all the mail one sees. After that, mail was sent by sea to Constantinople and then by land to Athos. Despite the fact that Athos was clearly part of Ottoman Turkey there seems to have been some confusion about mail franking with correspondents using Russian internal tariffs and then getting charged Postage Due - or not.
Most of the mail one sees comprises Money Letters with elaborate addresses and manuscript markings not always easy to interpret. So I was pleased to come across a piece of mail which at first sight could hardly be simpler, though it does soon get more complicated: