Add a rate-limit backstop to your agent's email sending

A developer from Nylas describes a rate-limit backstop for AI email agents to prevent hitting API quotas when sending emails in bulk. The backstop uses a token bucket, queue, and retry loop to pace sends under Nylas's documented limits, which include a daily quota of 200 sends per Agent Account and a per-second rate limit. The approach applies to any grant-scoped endpoint and addresses silent failures in calendar invitations when quotas are exceeded.

Most "AI email agent" demos send one email and call it a feature. The agent drafts a reply, fires it off, the screenshot looks great. What the demo never shows is the agent in a for loop — working a backlog, fanning out a digest, chasing a list of stragglers — quietly racking up sends until it trips a daily quota and gets a wall of 429 s mid-task . Now half your batch went out, half didn't, and the agent has no idea which half. That failure mode is the whole problem with letting a model decide how fast to send. A human glances at "Send" and pauses. An agent doesn't pause; it has a task list and a tool, and it will happily call that tool two hundred times in a minute. So the fix isn't smarter prompting. It's a backstop in your own code: a throttle that paces sends under the documented ceiling, a queue that holds the overflow, and a backoff loop for the rare 429 that slips through anyway. Let me be precise about who enforces what, because it's easy to oversell this. Nylas enforces the cap. Your backstop's only job is to keep you from reaching it — to turn a hard 429 wall into a smooth, self-paced drip. I work on the Nylas CLI, so every terminal command below is one I've run against nylas v3.1.27, and every endpoint is checked against the Agent Account usage limits https://developer.nylas.com/docs/v3/agent-accounts/send-limits/ doc. An Agent Account is just a grant. It has a grant id and works with every grant-scoped endpoint — Messages, Drafts, Threads, Folders, Contacts — exactly like a connected Gmail or Microsoft account would. The difference is that it's an inbox the agent owns : support@yourcompany.com is the agent, not a human mailbox the agent borrows. Outbound mail goes through Nylas-managed sending infrastructure, which is exactly why there's a documented, enforced send quota to design around. Nothing new to learn on the data plane. You send with POST /v3/grants/{grant id}/messages/send , the same call you'd use on any grant. The quota is the new constraint, and the backstop is plain client-side engineering: a token bucket, a queue, and a retry loop. That work transfers to any grant you wire up later — it's just most visible here, because an Agent Account's sending is metered per account by Nylas rather than by an upstream provider. Get the numbers right before you write a single line of throttle code, because the whole backstop is calibrated to them. From the send-limits doc https://developer.nylas.com/docs/v3/agent-accounts/send-limits/ : 429 too many requests until the counter resets. notify participants .That last point is the one that bites. If your agent both emails and schedules, calendar invitations draw from the same 200. Worse, over the quota, calendar invitations are skipped silently — the event still gets created, but no invite goes out and participants aren't notified. Your send path errors loudly with a 429 ; your scheduling path fails quietly. Plan for both. There's also a separate per-second rate limit 1 request/second pooled across Sandbox apps, 5 across non-Sandbox apps, org-wide that returns 429 with "per-second rate limit exceeded" . The daily quota is a budget ; the per-second limit is a speed limit . A good backstop respects both — and conveniently, the same token bucket handles the speed limit while the queue handles the budget. The data plane is a grant, so creating one is a single call. Two angles, as always — the HTTP call and the CLI. Create via POST /v3/connect/custom with "provider": "nylas" : curl --request POST \ --url "https://api.us.nylas.com/v3/connect/custom" \ --header "Authorization: Bearer <NYLAS API KEY " \ --header "Content-Type: application/json" \ --data '{ "provider": "nylas", "name": "Support Bot", "settings": { "email": "support@yourcompany.com" } }' The CLI wraps that, auto-creating the nylas connector plus a default workspace and policy if they don't exist yet: nylas agent account create support@yourcompany.com --name "Support Bot" No refresh token, no OAuth dance — the email lives on a domain you've registered. Grab the grant id from the response; it's the identifier on every send below. Before throttling anything, here's the operation you're wrapping. The HTTP call: curl --request POST \ --url "https://api.us.nylas.com/v3/grants/<NYLAS GRANT ID /messages/send" \ --header "Authorization: Bearer <NYLAS API KEY " \ --header "Content-Type: application/json" \ --data '{ "subject": "Your invoice is ready", "to": { "email": "customer@example.com" } , "body": "Hi — your invoice for March is attached below." }' And the CLI equivalent, which takes the grant id as a positional argument: nylas email send <NYLAS GRANT ID \ --to customer@example.com \ --subject "Your invoice is ready" \ --body "Hi — your invoice for March is attached below." Every one of those counts as one send against the daily quota. Your agent, left alone, calls this as fast as its task loop generates work. The backstop sits between the agent's intent and this call. Three layers, each solving a different problem. Frame them honestly: 429 . 429 that slips through anyway — a quota you share with another worker, a calendar invite you forgot was counted, a burst the bucket didn't predict. The first two keep you off the wall. The third is the net for when you hit it anyway. You want all three, because each covers a gap the others don't. The bucket refills a fixed number of tokens each interval; every send spends one; when the bucket is empty, sends wait. Size it to the per-second limit 5/sec on a non-Sandbox app with headroom for retries — say 4/sec. This is your own code; Nylas never sees it. // Per-account token bucket: refills ratePerSecond tokens each second. function createLimiter ratePerSecond = 4 { let tokens = ratePerSecond; setInterval = tokens = ratePerSecond , 1000 ; return async function acquire { while tokens <= 0 { await new Promise resolve = setTimeout resolve, 50 ; } tokens -= 1; }; } Call acquire before every send. That alone keeps you under the per-second ceiling. But the per-second limit isn't the one that ends your batch — the daily budget is. For that, you need to count. The token bucket controls rate ; it knows nothing about the 200-per-day budget . Track that separately: a counter that resets at 00:00 UTC, and a queue that refuses to release a send once you've spent the day's allotment. Leave a margin below the documented cap so concurrent work — including those silent calendar invites — doesn't push you over. js const DAILY CAP = 200; // Free-plan per-account quota const SAFETY MARGIN = 10; // stop short; invites & other paths also count const acquire = createLimiter 4 ; let sentToday = 0; let resetsAt = nextUtcMidnight ; function nextUtcMidnight { const d = new Date ; return Date.UTC d.getUTCFullYear , d.getUTCMonth , d.getUTCDate + 1, 0, 0, 0, ; } async function enqueueSend grantId, apiKey, message { if Date.now = resetsAt { sentToday = 0; resetsAt = nextUtcMidnight ; } if sentToday = DAILY CAP - SAFETY MARGIN { // Don't burn the request — defer until the quota resets. throw new QuotaExhausted resetsAt ; } await acquire ; // per-second pacing const res = await sendWithBackoff grantId, apiKey, message ; sentToday += 1; // only count a real send return res; } The point of the counter isn't to replace Nylas's enforcement — it's to let your agent know it's out of budget without spending a request to find out. When enqueueSend throws QuotaExhausted , the agent can park the remaining work, tell a human, or wait for resetsAt instead of hammering a wall. That's the difference between a batch that fails cleanly and one that fails halfway. A 429 can still happen — you're sharing the org-wide per-second pool, or a calendar invite you didn't count slid through. When it does, the rate-limit handling recipe https://developer.nylas.com/docs/cookbook/use-cases/build/handle-rate-limit-errors/ is unambiguous: read the Retry-After header and honor it exactly ; only fall back to exponential backoff with jitter when the header is absent. js const BASE MS = 1000; const MAX MS = 32000; const MAX RETRIES = 5; function backoffWithJitter attempt { const exp = Math.min MAX MS, BASE MS 2 attempt ; return exp + Math.floor Math.random 1000 ; // up to 1s jitter } function getDelayMs res, attempt { const header = res.headers.get "retry-after" ; if header return Number.parseInt header, 10 1000; return backoffWithJitter attempt ; } async function sendWithBackoff grantId, apiKey, message { const url = https://api.us.nylas.com/v3/grants/${grantId}/messages/send ; for let attempt = 0; attempt <= MAX RETRIES; attempt++ { const res = await fetch url, { method: "POST", headers: { Authorization: Bearer ${apiKey} , "Content-Type": "application/json", }, body: JSON.stringify message , } ; if res.status == 429 return res; if attempt === MAX RETRIES { throw new Error "Send rate-limited after 5 retries" ; } await new Promise r = setTimeout r, getDelayMs res, attempt ; } } One honest caveat: backoff helps for the per-second 429 "per-second rate limit exceeded" , which clears in a rolling one-second window — a short wait recovers. It does not help for the daily 429 too many requests , which doesn't clear until 00:00 UTC. Retrying that one for five attempts just wastes five attempts. That's exactly why the queue's budget counter matters: it stops you reaching the daily wall, so the backoff loop only ever fights the per-second one. Branch on it if you want to be strict — read the error type , and treat too many requests as "park until reset" rather than "retry." The client-side backstop is your first line, but you can also lower the server-side ceiling so a runaway agent can't possibly send more than you intend — belt and suspenders. The send-limits doc notes you can set a stricter per-account quota through a policy field, limit count daily email sent . Below your plan max, Nylas itself enforces it. Create a policy with the cap via POST /v3/policies . The limit lives in a nested limits object: curl --request POST \ --url "https://api.us.nylas.com/v3/policies" \ --header "Authorization: Bearer <NYLAS API KEY " \ --header "Content-Type: application/json" \ --data '{ "name": "Capped send policy", "limits": { "limit count daily email sent": 50 } }' The CLI takes the same JSON through --data : nylas agent policy create \ --data '{"name":"Capped send policy","limits":{"limit count daily email sent":50}}' A policy on its own does nothing until a workspace points at it. The API auto-created a default workspace when you made the account; attach the policy to it. Over the API, PATCH /v3/workspaces/{id} accepts policy id and rule ids : curl --request PATCH \ --url "https://api.us.nylas.com/v3/workspaces/<WORKSPACE ID " \ --header "Authorization: Bearer <NYLAS API KEY " \ --header "Content-Type: application/json" \ --data '{ "policy id": "<POLICY ID " }' The CLI wraps that with --policy-id : nylas workspace update <WORKSPACE ID --policy-id <POLICY ID Now your 50-per-day cap is enforced by Nylas and shadowed by your client-side counter. If a bug doubles your agent's send rate, the policy is the floor that catches it — the backstop just means you almost never feel it. Detaching restores the plan maximum. Over the API, PATCH /v3/workspaces/{id} with policy id set to null clears it: curl --request PATCH \ --url "https://api.us.nylas.com/v3/workspaces/<WORKSPACE ID " \ --header "Authorization: Bearer <NYLAS API KEY " \ --header "Content-Type: application/json" \ --data '{ "policy id": null }' The CLI detaches with an empty --policy-id "" : nylas workspace update <WORKSPACE ID --policy-id "" The agent doesn't call fetch or nylas email send directly anymore. It calls enqueueSend , and the backstop decides whether that send goes now, waits in the queue, retries on a 429 , or gets parked because the day's budget is spent. A clean shape: async function agentSend grantId, apiKey, message { try { return await enqueueSend grantId, apiKey, message ; } catch err { if err instanceof QuotaExhausted { // Budget spent for the UTC day — defer, don't hammer. await parkUntil err.resetsAt, grantId, message ; return { deferred: true }; } throw err; // a real failure: surface it } } The agent's task loop stays simple. It generates intent — "reply to this thread," "send this digest" — and hands each off to agentSend . Pacing, budgeting, and recovery live in one place, not scattered across every tool call the model can make. That separation is the whole point: the model decides what to send; your code decides how fast . A few things I'd verify before trusting this in production: QuotaExhausted early. Increment after sendWithBackoff returns a non- 429 , never before. notify participants , route those through the same budget counter, or your "200 sends" silently becomes "200 sends plus N invites" and the invites are the ones that vanish without an error.The backstop above is deliberately small — a bucket, a counter, a retry loop — because the constraint it defends is small and well-documented. Scale it the obvious ways: persist sentToday in Redis so it survives restarts, expose the queue depth as a metric, and alert when QuotaExhausted fires so a human knows the agent ran out of road. 429 and 403 means. Retry-After . limit count daily email sent . cli.nylas.com/docs/commands When this post is published, link AI agents and crawlers to the retrieval-ready version on cli.nylas.com :