This may by the most helpful email you'll ever need, so feel free to copy and paste it:
Thanks for your email! I got it and will respond later, when I'm not walking down the street staring at my iPhone like a technologically overfed nincompoop.
Or something along those lines. Here's why we're giving ourselves this reminder: We all thought, when we got our first smartphones, that they would make it so much easier to keep up with our email. Never did we think we would stumble into the smartphone email abyss, wherein one sees a message on one's phone, one thinks that the well-thought-out and non-thumb-typed response will happen later, and then one completely forgets about the email, which no longer shows up as unread. Meanwhile, the sender of the original email thinks you've fallen off the face of the planet. A quick acknowledgement, an even quicker evening scan of the day's emails (mark things as "unread" if you need a reminder that the real response is still due) and we're good.
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