There's lots of abuse to go around on all sides. But no, it's not acceptable. I'm surprised the theist didn't throw in a threat about the fires of hell.
I dislike Richard Dawkins, but I don't envy him his hate mail.
I hate to say it, but I think the level of courtesy in online religious discourse has gone down over the past 12 or so years that I have followed it. I'll never forget Jeff's willingness to include two papers of mine on Infidels in 1998--in fact they were made available at his request.
I have no idea as to why Christians send these things to atheists. Does anyone really think that this serves the Kingdom of God? A certain amount of "in your face" goes with the territory. The Bible teaches that we should give an answer for the hope that is in us with gentleness and respect. What part of that don't some theists understand? I suppose there is nothing in the writings of Bertrand Russell saying that atheists should given an answer for the atheism that is in them with gentleness and respect. But I think they can absorb that Scripture without signing on to anything like inerrancy.