Hi, This is a 2 part question. 1- If a right handed kohen does the avoda with his left hand by mistake, is the korben posul? 2- What is the ’mum’ of being a lefty? If a lefty trains himself to become a right handed, does he go out of mum status?

ב"ה Shalom It is quite refreshing to hear a question which involves the return of the Kohanim to do their Avoda-=service in the Bet Hamikdash. It reminds us of what we are to expect in the days to come. I will base my answer primarily on what I found in the Gemara and Rambam. The Gemara (בכורות מג:) distinguishes between a "Bal Mum", someone who has a blemish and someone who is dissimilar to the rest of the seed of Aharon"-אינו שוה בזרעו של אהרון. The Kohen who is dissimilar, although unfit for Avoda is not considered a "Bal Mum. "Meaning it is considered an irregularity, but not a "mum". The difference between the two being, that the "Bal Mum. ", if he went ahead and did avoda, it is desecrated , unlike in the case of the "dissimilar", if he went ahead and did avoda, it is not desecrated . The Rambam (הלכות ביאת מקדש פ"ח הלכות יא ויג) enumerates a left handed person as one disqualified for Avoda, but on the other hand (הלכות ביאת מקדש פ"ו ה"ו) wrote that the disqualifications which are exclusive to humans, a person who did Avoda is indeed punishable by Makot, but his avoda is not disqualified. This would be because the disqualification is not because he is a "Bal Mum" but because he is " dissimilar to the rest of the seed of Aharon." Rashi (בכורות מה:) says that the disqualification of a left handed person is because the Torah distinctively wrote that Avoda must be done by the right hand, meaning that it is not because he is a Bal Mum. Furthermore, the Rambam (הלכות ביאת מקדש פ"ח הלכות יא ) writes based on the opinion of Chachamim in the Gemara (בכורות מה:) that an ambidextrous is qualified for Avoda. Although it may not be applicable in your case,if you were to ask me, should a left handed Kohen be trained to use his right, based on what Rav Yitzchak Zilberstein Shlit"a wrote in his commentary חשוקי חמד, to Bechorot מג:, it seems that if this would affect his Torah study , it would not be recommendable. May we see the building of the Bet Hamikdash speedily in our time. All the Best