Asking for Editor comments even before submitting to the journal, Is it ethical? I had written a manuscript and had got it verified by a Professor (as it was closely related to some of his work) who said it was correct. I later came to know that he is also an editor of a reputed journal. So I Wrote to him (the Editor of a journal (math)), asking to comment about suitability of the manuscript for that journal, even before submitting to the journal, is it ethical? and not something frowned upon?

PS : I have not submitted to that Journal.
I don't believe there is anything unethical in asking. You are just getting an opinion, if anything. Of course, the editor may decline as it may raise ethical issues on that end, but not on yours. You aren't asking for an advantage over others, but just advice on suitability. That is the editor's function, though they may prefer, and have rules, to only comment on proper submissions. 

He has seen the paper already, of course. Some editors would actually encourage you to submit it without even being asked. It will be judged by reviewers other than the editor, of course.