I understand that "I haven't heard from him until now" means "I didn't hear from him before and I just hear(d) from him now".

But what about "I didn't hear from him until now"? Does it mean the same as the previous sentence, or "I haven't heard from him ever, not even now"? Is it grammatical and does it sound natural? 

By the way, can I replace "until now" with "till now" or "up to now" in those sentences? According to [this Q/A][1], "until" is preferred for negative conditional sentences, but I am not sure if it's relevant to my question.


  [1]: https://english.stackexchange.com/questions/6989/what-is-the-difference-between-till-and-until