Просто, чтобы не потерять

～だろう = "I think (some clause)." The speaker thinks the clause is probably (or almost certainly) true.
～ではないだろうか = "I think (some clause), isn't it?" The speaker thinks the clause is perhaps true, but he's less certain as compared to the first sentence.
～だろうか = "I wonder whether or not (some clause)." The speaker doesn't know if the clause is true or false. You cannot add 恐らく to this sentence because the speaker is totally unsure.
～ではないだろう = "I don't think (some clause)." The speaker thinks the clause is probably not true.

彼は（恐らく）大丈夫だろう。 He must be okay.
彼は（恐らく）大丈夫ではないだろうか。 I think he is okay.
彼は大丈夫だろうか。 I wonder if he is okay. / I'm worried if he's okay.
彼は（恐らく）大丈夫ではないだろう。 He must not be okay.

In the example sentence, the speaker is saying "It's probably impossible (無理 to use e-mails in place of ...", so 恐らく無理ではないだろうか is the correct phrase. 恐らく無理だろう is equally fine. The existence of the word 恐らく is what makes ～だろうか wrong. You can't usually say "I wonder if he is probably fine." in English, either.