email sender delux will do it from your own PC but it will take along time to send 500k. might want to batch them into smaller chunks.
mail chimp also has a free service but I dont know if they will let you import those emails.
I highly doubt you have legitimately collected 500k emails if you dont have any money to spend on email software. Any business that has collected that many emails must have some $$. So careful about sending out spam my friend. Thats probably what mail chimp will say if you try to import those emails.