According to Prof. Orah Limor in her book in Hebrew "Between Jews and Christians: Jews and Christians in Western Europe Until Modern History", Vol. 3, pg. 87, most scholars believe that the epistle was written by a Christian named Alphonsus Bonihominis (Alphonso Buenhombre in Spanish), being that according to the introduction to the epistle, he was the person who translated it into German. According to the introduction, he was a 14th-century Christian-from-birth Dominican monk who allegedly had been appointed as Bishop of Morocco. Again according to him, the original epistle in Arabic found its way to him when he was in a Saracen prison.
According to Limor in her English essay on the epistle, the work and a Jewish polemic work against it were mentioned in the introduction to the Meir Nativ Concordance. A non-critical but well-punctuated (and, perhaps more importantly, with a clearer font) copy of the introduction (which, as it turns out, only appeared in the first printed edition) was brought in Ram Ben Shalom's essay "Meir Nativ: The First Hebrew Concordance of the Bible and Jewish Bible Study in the Fifteenth Century, in the Context of Jewish-Christian Polemics", pp. 335-364. The part of the polemic is mentioned on p. 356: "אמנם, כבר המצאתי בהצעת תשובותי לאגרת פיקורי ר' שמואל...".
Translation: "Although, I have already brought in my suggested answers to the Epistle of the Heresies of R' Shmuel..."
As you can see, he added the term "פיקורי" - "heresies" to the title of the epistle. In any case, we can see here that he mentions a work that answers the points made by Buenhombre, and as he saw fit to mention it in the introduction to his concordance, we may assume that it had spread at least somewhat around the Jewish communities. This work, sadly, is no longer extant, to my knowledge.
Encyclopedia Judaica, Second Edition writes the following about him:
"°ALFONSUS BONIHOMINIS (Buenhombre; d. 1353), Spanish Dominican, born in Cuenca or Toledo. From a stay in Morocco, where he had been imprisoned, Alfonsus claimed to have brought back the Arabic original of the De adventu Messiae, an anti-Jewish epistle allegedly written by one Samuel of Fez. He said that he had translated this text in Paris in 1339. Known as the “Epistola Samuelis Maroccani,’ it was later translated into several languages and widely circulated in Europe. In fact, it seems that he himself was the author, drawing largely from another tract in Arabic written by a Jewish convert to Islam, *Samawal b. Judah ibn Abbas, probably with the intent of presenting it as a Christian rather than a Muslim polemic. Alfonsus also translated another Arabic treatise by Samuel (or possibly wrote it himself): Disputatio Abutalib Saraceni et Samuelis Judaei quae fides praecellat: christianorum, an iudeorum, an saracenorum (Ms. Madrid Nac. 4402, fol. 103-10), a disputation between a Saracen and a Jew."
Encyclopedia of the Middle Ages writes this about him:
"ALFONSO BUENHOMBRE (late 13th c. - 1353), Alfonso Buenhombre (Alfonsus Bonihominis), a *Dominican missionary and polemist, born perhaps at *Toledo, led an adventurous life that took him notably to Egypt (1336), Morocco (1337-1338) and Famagusta (1341) he was elected bishop of Morocco in 1344. His travels (and perhaps studies in one of the Dominican studia linguarum?) gave him a good knowledge of Arabic, allowing him to translate several texts: an apocryphal History of the patriarch Joseph (1336), a Life of Sr Anthony the Hermit (1341). He also presented as translations from Arabic a Disputatio Abutalib Sarraceni et Samuelis Iudei on the best religion (1339-40) and his best known work, extremely widespread in the 14th c., the Epistola rabbi Samuelis (1339); in the latter case this is probably a fiction. This Epistola is an apology for the Christian faith, written in the character of a Jew addressing a co-religionist. We also owe him a Tractatus contra malos medicos (1342)."