---
title: What email actually costs when your agents send it
metaTitle: The cost of email for AI agents, worked through
description: Per-message pricing, per-inbox pricing, storage, and the hidden costs nobody budgets for: tokens spent parsing mail, engineering time on threading, and the price of a damaged sending domain.
date: 2026-07-27
author: MCPmailer
tags: Operations
---

Email looks cheap per message, which is why people budget for it badly. The line item is small and the surrounding costs are not: the tokens spent reading threads, the weeks spent building the parts a mail API does not give you, and the occasional expensive month where a sending domain has to be rebuilt.

![Message volume, inbox count, and storage as the three axes of an email bill](/blog/cost-of-email-for-ai-agents/hero.webp "Three things drive the bill: how much you send, how many identities you keep, and how much you store.")

## What you are billed for

Three axes, and most people only think about the first.

**Messages.** Sends draw down a monthly allowance, and both replies and cold sends draw down the same one. On MCPmailer that is 3,000 a month on Free with a 100 a day ceiling, 10,000 on Pro, and 150,000 on Startup, with overage metered at $0.001 an email on Pro and $0.0006 on Startup.

**Inboxes.** An identity per agent is the correct architecture, so the number of agents is a cost line. Three are included on Free, ten on Pro and then a dollar each, 150 on Startup.

**Storage.** Mail with attachments accumulates. 3 GB on Free with 30 day retention, 10 GB on Pro, 150 GB on Startup, and ten cents a gigabyte beyond that. Retention is unlimited on paid plans, which is a feature and a bill: archive what you have dealt with.

Current numbers and the full comparison are on the [pricing page](/pricing), also available as [markdown](/pricing.md) if you would rather your agent read it.

## Working an example

A support agent answering 40 threads a day, roughly 1.2 replies each, is about 1,400 messages a month. One inbox. Attachments occasional. That fits inside Pro with room to spare, so the marginal cost of the agent is the plan, not the mail.

Now add a billing agent doing 600 chases and a scheduling agent doing 900 coordination messages. Three inboxes, about 2,900 messages, still inside Pro. This is the shape most teams land on: the cost is dominated by the plan and the agent count, not by volume, until volume becomes genuinely large.

At 150,000 messages a month you are in Startup territory, where overage is cheaper and the interesting question changes from price to whether that volume is conversational at all. Five recipients per message is a hard cap, and campaign-shaped sending belongs on campaign infrastructure.

| Deployment | Inboxes | Messages a month | Where it lands |
| --- | --- | --- | --- |
| One agent, evaluation | 1 | Under 3,000 | Free |
| Small fleet in production | 3 to 10 | Under 10,000 | Pro |
| Product with an inbox per tenant | 10 to 150 | Tens of thousands | Pro with inbox overage, or Startup |
| High-volume conversational | 150+ | 150,000+ | Startup or a conversation with us |

## The costs that are not on the invoice

**Tokens.** This is usually the larger number and it is entirely within your control. A fifty message thread fed to a model whole, with quoted history, costs multiples of the same thread stripped and summarised. `get_thread` returns conversations without the quoted repetition for this reason, and extracting the fields you need from an attachment beats pushing a contract through a model every time the thread is revisited. See [attachments](/blog/email-attachments-for-agents).

**Engineering.** The parts a raw mail API leaves you: threading, quoted-history stripping, waiting for replies without a scheduler, suppression on bounce, idempotency under webhook retries, an oversight surface a non-technical colleague can read. That is weeks, and then it is maintenance forever. The comparison of building on IMAP or a provider API is in [agent inbox or shared mailbox](/blog/agent-inbox-vs-shared-mailbox).

**Reputation.** The expensive one. A sending domain that gets filtered costs a quarter to recover, because reputation attaches to the domain and you cannot buy trust for a new one. Everything in [deliverability for agent senders](/blog/agent-email-deliverability) and [the warmup plan](/blog/email-warmup-plan-agents) is cheaper than the alternative.

**Bad answers.** A confidently wrong reply costs a support contact, sometimes a customer. This is why [evaluation](/blog/evaluating-ai-email-agents) is a cost control and not a nicety.

![Invoice cost beside token cost, engineering time, and reputation risk](/blog/cost-of-email-for-ai-agents/hidden.webp "The invoice is usually the smallest of the four.")

## Keeping the bill predictable

- **Set a monthly spend cap.** Available on paid plans, and the single best protection against a loop that discovers it can send.
- **Archive aggressively.** Storage is the line that grows quietly, and archiving keeps `list_messages` useful at the same time.
- **Count inboxes deliberately.** One identity per agent is right; one per experiment that nobody turned off is not.
- **Watch tokens per thread,** not just messages per month. Long threads and whole attachments are where the real money goes.
- **Alert on volume changes,** in both directions. A spike is a runaway; a collapse is a broken webhook.

## Free is a real tier, with a real edge

Three thousand messages a month, 100 a day, three inboxes, and 30 day retention is enough to build the thing, run it against real mail for a fortnight, and decide whether it works. The daily ceiling is the constraint that bites first: an agent answering a busy inbox will meet 100 a day before it meets 3,000 a month, and the refusal says `daily_send_quota_exhausted` with the reset time rather than failing silently.

Move to a paid plan when the daily ceiling starts refusing work you wanted done, not before.

## Questions

### How much does email for AI agents cost?

It is driven by messages, inboxes, and storage. Free covers 3,000 messages a month across 3 inboxes, Pro is $20 a month for 10,000 messages and 10 inboxes, and Startup is $200 for 150,000 messages and 150 inboxes, with metered overage beyond. See [pricing](/pricing).

### Do replies cost the same as cold sends?

Yes, both draw down the same daily and monthly allowance. The difference is behavioural: a cold send carries an unsubscribe link and is watched by the duplicate-content and velocity tripwires.

### What is the biggest hidden cost?

Usually tokens, then engineering time on the parts a raw mail API does not provide. The most expensive rare event is a damaged sending domain, which takes months rather than money to fix.

### How do I stop an agent running up a bill?

Set a monthly spend cap, and alert on volume changes. A refused send returns `monthly_spend_cap_reached`, which the agent can act on rather than retry.

### Does an inbox per agent get expensive?

At a dollar per additional inbox on Pro it is rarely the dominant cost, and the alternative, several agents sharing one identity, costs you attribution, scoping, and clean revocation.

### Is there a free tier worth using?

Yes, and it is sized for exactly the right thing: building the agent and running it against real mail for a couple of weeks before committing.

## Related

- [Pricing](/pricing)
- [Deliverability for agent senders](/blog/agent-email-deliverability)
- [Evaluating an email agent](/blog/evaluating-ai-email-agents)
- [Agent inbox or shared mailbox](/blog/agent-inbox-vs-shared-mailbox)