If someone in your life ever takes the time to send you an email, then you should bother to reply, even if it is only to acknowledge receipt of their message.
(This is an edited repost from my professional blog, because this bears repeating.) I have a spam problem, so much so that I shut comments down on my blogs years ago.
I spend most of my working day reading, writing, sending, and managing email — so the tool I use must be as efficient as possible. When I was on Windows, I was an Outlook power-user. I really came to hate Windows back then so switched over to Mac. I loved everything about my new Mac experience… except I missed Outlook.