Email Triage Taxonomies for LLM Classification

The most important design decision in an email classifier isn't the model — it's the label set, and here's the one I keep coming back to: You triage email into one of four categories: URGENT — production incidents, executive requests; reply within 1 hour ACTION — code reviews, meeting follow-ups; reply same day FYI — informational, no response needed NOISE — newsletters, marketing, automated notifications From: {sender} Subject: {subject} Snippet: {snippet} Return ONLY the category name. Nothing else. That's the working prompt from the Nylas email triage recipe , and almost every line encodes a taxonomy-design lesson worth unpacking. Most people building email agents obsess over model choice and prompt phrasing. The recipe's quiet thesis is that the label set itself does the heavy lifting — get the taxonomy right and a cheap model classifies well; get it wrong and no model saves you. Why four is the magic number The recipe states it flatly: four is the right number. Three loses fidelity — everything important collapses into one overloaded bucket and you've built a binary classifier with extra steps. Five and the model starts confusing categories, because the boundaries between adjacent labels get too thin to express in a definition. Notice what makes these four work. They aren't topics — they're response obligations . URGENT means "reply within the hour," ACTION means "reply today," FYI means "no response needed," NOISE means "archive." Each label maps to exactly one behavior. That's the test I'd apply to any email taxonomy: if two labels lead to the same action, merge them; if one label leads to two different actions depending on content, split it. The same principle shows up in the sales context. The Agent Accounts overview describes an outreach agent classifying replies as interested / not now / unsubscribe — three labels, because the workflow has exactly three branches: book the meeting, schedule a follow-up, stop emailing. The taxonomy is the decision tree, fla