I think you've answered your own question. There are non-Jews who are E-M123 (plenty of them, especially in the Levant), so it's not "necessarily Jewish". However, Poland did historically have a large Jewish population, & M123 is not a very common haplo in Poland. So I'd say these factors weigh in favor of it being Jewish in origin. Almost all the indigenous E in Poland is E-V13 (4%). However it is certainly possible that the E-M123 came from the south in Greece or around the Black Sea, or came from Italy or the Caucusus or something. This can't be ruled out at this stage. There has been plenty of gene flow into Eastern Europe from those regions too. It should also be noted that ethnic Poles possess the marker in very small amounts (<.5%). You can do a more penetrative test & find out for certain if your line is essentially an exclusively Jewish one. Or you can compare your DNA to folks online to save yourself a little money. This might give you a pretty good idea regarding the likelihood your Y-DNA comes from a Jewish progenitor. But M123 is not exclusively Jewish in Europeans by any means. Napoleon was E-M123.