[kingsinger]

Sounds like a reasonable approach. That said, if the message requiring a response doesn't have issues in it that are time sensitive (e.g., it's more like a personal letter from the old days where somebody is saying hi and updating you on the events in their life), you might consider viewing their message as two tasks:

(a) a short GTD less than two minutes do it now e-mail back thanking them for their message and letting them know that you hope to get back to them with a longer reply when you have the time/cognitive space to do that; and

(b) the longer message you write at a later time.

I think most people understand when somebody is busy/overwhelmed, etc. But they also appreciate at least getting a timely message back, even if it's just acknowledging receipt of their message.

That gesture alone says a lot, and I think helps to maintain connections with people across boundaries and time.

Remember, communications with friends don't always need to be perfect. If you hold yourself to that standard all the time, you just won't respond at all. Then, your relationships will suffer far more than if you had just made a point to send back a quick response.