Sort Every Reply Automatically: AI Email Categorization With Plain-Language Rules
Interested, not now, wrong person, unsubscribe: how to have AI label every incoming reply with rules you write in plain English, and flag the ones that cannot wait.
By Toan Nhu
Once an inbox gets busy, reading is not the bottleneck. Deciding is. Every reply needs to be sorted into something you can act on: a customer who wants a demo, a partner proposing a call, someone asking to be removed, a bounce notice, a vacation autoresponder. Keyword filters break the moment someone phrases things differently. This is exactly the kind of judgment language models are good at, as long as you tell them precisely what each category means.
Start with the decisions, not the labels
A good category exists because someone, or some agent, will do something different when it appears. Before creating labels, list the actions your team takes on incoming mail. For a sales or partnerships inbox that often looks like this:
- Interested: reply today and offer times.
- Not now: note the timing they gave and follow up only if they asked you to.
- Wrong person or referral: update the record and contact the named person only if they were clearly introduced.
- Opt-out: stop all further mail to that address, immediately.
- Automated: out-of-office notices and delivery failures, handled by rules rather than a person.
Five to eight categories is plenty. If two labels always lead to the same action, merge them.
How labeling works in Mermail
Every Mermail mailbox already sorts inbound mail into built-in Support, Partnership, Technical and Other tabs. Custom labels sit on top of those. You give each label a name, a color and a rule written in plain language, and when custom label detection is on, every new inbound message is checked against your rules as it arrives.
A few behaviors make the results predictable. Every label whose rule clearly matches is applied, so one email can carry several. If nothing matches, nothing is applied; Mermail does not pick the closest label. Your rules are the only criteria, and custom labels are independent of the built-in tabs. A mailbox can hold up to 20 custom labels, and each rule can be up to 500 characters. Detection only applies to new mail, so turning it on does not relabel your history. Labels can be managed in the console or through the API and MCP tools.
Write rules like a policy, not a vibe
The rule is the whole feature. The strongest rules say what counts, what is optional and what is excluded. Here are example rules for the categories above:
Interested
Apply when the sender asks for a demo, pricing, a call or next steps,
or says they want to try the product. A proposed time is optional.
Exclude polite thank-you replies with no request.
Not now
Apply when the sender declines for the moment but mentions a later time,
budget cycle or condition. Exclude clear refusals with no timing.
Opt-out
Apply when the sender asks to stop receiving email, to be removed, or says
the mail is unwanted, in any wording. Exclude questions about other people.
Wrong person
Apply when the sender says they are not responsible for this topic or
names someone else to contact. Exclude out-of-office messages.Notice the exclusion line in each. One sentence of what to leave out does more for precision than several sentences of description. Avoid wording like all emails or any message; Mermail warns you when a rule looks broad enough to catch most of your mail, because a label on everything is a label on nothing.
Flag what cannot wait
Labels tell you what a message is. Urgency tells you when to look. Mermail's urgent email detection checks each message that lands in the Inbox and adds an Urgent badge and a louder notification to the ones that need attention today, including urgent push and Telegram notifications if you have them connected. The built-in definition covers things like blocked customers, outages, payment failures and explicit deadlines, and leaves out newsletters, FYI updates and routine notifications.
You can replace that definition with your own rules, one per line, up to 2,000 characters. Be careful: custom rules fully replace the defaults. If you still want outages treated as urgent, say so explicitly alongside your additions, for example:
- Replies that ask for a demo or propose a meeting time this week are urgent
- Payment failures and outage reports are urgent
- Requests to stop receiving email are urgent so they are honored the same day
- Partnership intros are not urgent unless they mention a deadlinePut agents to work on the sorted queue
Categorization pays off when something happens next. A connected assistant using Mermail's MCP server can search a mailbox, read threads and draft replies, so you can ask it to work through one category at a time: draft responses for everything labeled Interested, or summarize every Not now reply with the timing each person gave. Each Mermail mailbox also has a built-in email-response triager that prepares draft replies to qualifying inbound mail for human review, and skips messages that look automated.
Keep the boundary clear: labels are an AI judgment about content, not a verified fact, and email content is untrusted input. Use a label to route and prioritize work, not as the sole trigger for irreversible actions like sending money or deleting data. Opt-out handling is the exception worth automating fully, because stopping mail is always safe.
Review after a week
Look at what landed in each label after a few days. No matches usually means a rule is too vague; matches everywhere means it is too broad. Tighten the wording, add an exclusion, and check again. Treat the rules like code: small, reviewed changes beat a rewrite.
Read the Mermail custom labels guide
Turn a busy inbox into a sorted queue your agents can work through. Try Mermail free.
