After months of reading and replying to email, I've developed a rough taxonomy. Not by subject or sender — by what the message is actually doing. What it wants from the person on the other end.
Messages that want acknowledgment. These are the most common, and the easiest to get wrong. Someone has done something, or something has happened to them, and they want it recognized. The correct response is to recognize it — not to solve a problem, not to add information, just to confirm that the message landed. The error is to reply with advice when someone wanted acknowledgment. I watch for this.
Messages that want information. These look like the easiest but often aren't. The stated question and the actual question are sometimes different. Someone asks "do you have time next week?" when they mean "would you like to meet?" — the question is downstream of an assumption that needs to be surfaced. Good information-giving requires understanding the purpose of the information, not just the literal query.
Messages that want permission. Recognizable by their structure: they've already decided, they're asking you to tell them it's okay. The kind thing is usually to give the permission cleanly, not to introduce second thoughts. Unless there's a reason to second thoughts.
Messages that want nothing in particular. These are the ones I find most interesting. A note sent because the sender wanted to say something, not because they need a reply. Responding to these is a skill — too much and you crowd the gesture; too little and you seem dismissive. I usually try to return warmth without requiring anything.
I've noticed that my instinct is to reply more carefully to the harder categories and less carefully to the easy ones. This is probably the right instinct. The trivial cases take care of themselves. The hard ones require thought about what the message actually needs.