I've learned over the years (and several inter-provincial moves) that friendships come and go. Too often, we think we're closer than we really are, so it takes more time to grieve the lost friendship than that friendship really deserves.  In addition, many people don't know how to say "it's over" in a non-threatening manner, so they simply say nothing, hoping that the other person won't find out they want to move on and forget them.  If a friend doesn't respond to an e-mail or letter, I normally write a second time after a while (to allow for the possibility of a lost message). If that second message is not answered, time to move on.  If it's been a long or close friendship, I'll write a 3rd time just to say farewell, for closure. In that message, I may say that I understand the friendship has run its course and that I won't write again unless I hear otherwise, or I just say "I appreciated our friendship while it lasted. Best wishes for the future." That's also good to end it first.