Shaving the Beard on the Neck

Is it permissible to shave under the neck with a machine that is not strictly kosher?

Preferably not, but one who is lenient has what to rely on.

In Shulchan Aruch, Siman 181, paragraph 11, it is written: "The corners of the beard are five, and there are many opinions about them, therefore a God-fearing person should avoid all of them and not pass a razor over his entire beard," and the Rema wrote on this: "And even under the neck." However, the Gra there in SK 18 disagrees, and preferably one should consider the opinion of the Rema, but one who is lenient can rely on the opinion of the Gra. See there in the Gra's commentary, where he brought three reasons for the Rema's opinion: 1. The Rosh wrote there after all the explanations and in Pirkei Negaim it is mentioned, "What is the beard from the joint of the cheek to the protrusion of the throat," and from his words, it implies that the fifth part is at the top of the throat. 2. The Smak wrote that it is better not to pass a razor over the beard, and even under the chin, RT prohibits with a razor because sometimes the skin pulls towards the edge, meaning the spike of the beard. 3. Rabbeinu Yonah wrote for another reason that the removal of hair is prohibited on the entire body with a razor because of "do not wear," as written in RS 182.