30-50 years is surely "forever" even in snail mail terms. 1000 years? Wow now we're talking about an era-defining technology for civilization itself! I'm going

30-50 years is surely "forever" even in snail mail terms. 1000 years? Wow now we're talking about an era-defining technology for civilization itself! I'm going to go with your lower bound: if email lasts largely unchanged for 30 years, then it will have been the most important application of the internet era.