Does this happen with ALL
replied to emails?
And you didn't mention what method you used to migrate your Outlook email to Mail, and personally I find that Apple's Mail option is not always the best.
It's pretty easy to select and delete such extra space when replying and have you checked your Mail > preferences > accounts > composing tab etc. settings? Maybe something in there that's causing the problem?
Or, maybe this old fix still works:
Hawk Wings » Blog Archive » Apple Mail: “Hacking” the reply string
If none of those work for you and you want things to 'just work normally
', then maybe archive and save the 'goofy' Mail account somewhere, then delete the goofy Mail account and start over by creating a new Mail account. Even if that's a bit of a drastic solution for just getting rid of some extra line feeds with some or any email replies.
Edit: Are you sure the sender of such emails haven't just added some extra line feeds to those emails?
Using TextEdit, or TextWrangler should show if any extra line feeds/carriage returns had been added to their original email that Mail would recognize and respect.