agent email APIs compared: lobstermail vs sendgrid vs resend

If you're wiring up email for an AI agent, the first instinct is to reach for a familiar API. SendGrid, Resend, Mailgun. You've used them before. They have great docs, solid SDKs, and battle-tested infrastructure for sending transactional email.

But here's the thing: most email APIs were built for a world where a human sets up the account, a human configures the domain, and the application sends outbound messages. Newsletters, password resets, order confirmations. The email flows one direction: out.

Agents need something different. An agent that handles support inquiries needs to receive email. An agent that signs up for services needs its own address to catch verification codes. An agent that coordinates with other agents needs a persistent inbox, not a webhook that fires and forgets.

This isn't a "which is best" post. These are genuinely different tools built for different problems. But if you're building for agent use cases, understanding where each one fits will save you from architectural dead ends.

What traditional email APIs do well#

SendGrid and Resend are excellent at what they were designed for: high-volume outbound email with strong deliverability.

SendGrid has been around since 2009. It processes billions of emails per month, offers granular analytics, IP warmup tooling, and a mature webhook system for tracking opens, clicks, and bounces. Their free tier gives you 100 emails per day. The Essentials plan starts at $19.95/month for 50,000 emails. If you need to send a lot of email reliably, SendGrid is proven infrastructure.

Resend launched as a modern alternative with a developer experience that's hard to beat. Clean API, React Email integration for building templates in JSX, and a straightforward SDK. Their free tier covers 100 emails per day (3,000/month), and the Pro plan runs $20/month for 50,000 emails. If you care about DX for outbound transactional email, Resend is a strong choice.

Mailgun sits somewhere in between, with solid outbound capabilities plus inbound routing. You can configure routes that forward incoming email to webhooks based on pattern matching. Their Flex tier is free for 100 emails/day, and paid plans start at $35/month.

All three handle SPF, DKIM, and DMARC. All three have good SDKs. All three will get your transactional email delivered.

Where they fall short for agents#

The gap shows up the moment your agent needs to do more than send.

No persistent inboxes. None of these services give your agent an inbox it can query. SendGrid has Inbound Parse, which forwards incoming email to a webhook endpoint as a POST request. That's useful for processing inbound messages in real time, but there's no stored mailbox. If your webhook is down when the email arrives, it's gone. Resend has no inbound capability at all. Mailgun has inbound routes with similar limitations to SendGrid's approach.

No agent self-signup. To use any of these services, a human creates an account, verifies a domain, generates API keys, and stores them in environment variables. The agent can't provision its own email address. If you're building autonomous agents that need to spin up their own communication channels, you hit a wall immediately.

No inbox querying. An agent processing a support queue needs to poll for new messages, filter by sender, and fetch full message bodies on demand. Traditional email APIs don't expose this. You'd need to build a storage layer on top of inbound webhooks, handle deduplication, manage retention, and build your own query interface. That's a significant amount of infrastructure for something that should be a primitive.

No receive-first architecture. These APIs assume sending is the primary action. Receiving is an afterthought bolted on through webhook parsing. For an agent whose main job is reading and responding to incoming email, the architecture is inverted.

Where LobsterMail fits#

LobsterMail was built for the use case where the agent is the primary user of the email address.

The agent calls LobsterMail.create() and gets a working inbox. No human account creation, no API key management, no domain verification step. The agent provisions what it needs and starts receiving email immediately. If you want to understand how this works mechanically, we wrote about agent self-signup in detail.

Receiving is the default. On the free tier, your agent gets an inbox at @lobstermail.ai and can receive email at no cost. The API supports polling with cursor pagination, since filtering, and lazy-loaded message bodies from storage. Webhooks are available for real-time push when polling isn't fast enough.

Sending unlocks on the Builder tier at $9/month. You get up to 1,000 emails per day, 10,000 per month, per-account reputation isolation via dedicated SES config sets, and pre-send validation that checks recipient format, MX records, and suppression lists before anything leaves.

The SDK exposes email.safeBodyForLLM(), which wraps incoming message content in boundary markers and strips potential injection attempts before the content reaches your agent's context window. That's specific to the agent use case and something you'd otherwise need to build yourself on top of any traditional API.

The comparison at a glance#

When to use what#

Use SendGrid or Resend when your application needs to send transactional email at scale. Password resets, order confirmations, marketing campaigns, notification emails. The human sets up the account, the application sends through the API, and the recipient is a human reading their inbox. This is what these tools were built for, and they're very good at it.

Use Mailgun when you need outbound sending plus basic inbound processing. Mailgun's routes let you match incoming email by recipient pattern and forward to webhooks. It works for use cases where you need to parse replies or process incoming messages, but don't need persistent storage or inbox querying.

Use LobsterMail when the agent is the one with the email address. When the agent needs to receive email, query its inbox, and respond on its own terms. When you're building multi-agent systems where each agent needs its own address. When you don't want a human in the loop for provisioning. When receiving is the primary operation, not an afterthought.

These categories aren't mutually exclusive. You might use Resend for your application's transactional email and LobsterMail for your agents' inboxes. Different tools, different jobs.

A note on combining services#

A common pattern we see: developers use a traditional API for their application's outbound email (welcome emails, receipts, notifications sent to humans) and LobsterMail for their agents' inboxes (receiving customer inquiries, processing verification emails, coordinating between agents).

This makes sense. SendGrid excels at sending millions of marketing emails with deliverability analytics. LobsterMail excels at giving each agent on the reef its own inbox where it can receive, read, and respond. There's no reason you can't use both in the same system.

The mistake is trying to force a send-focused API into a receive-focused use case. Building a persistent inbox layer on top of SendGrid's Inbound Parse is possible, but you're rebuilding infrastructure that already exists as a first-class feature elsewhere.

If your agent needs to send and receive, and you want it to manage its own email without human setup, that's the specific problem LobsterMail was designed to solve. If your application needs to send high-volume transactional email to humans, reach for the tool that's spent a decade optimizing for that.

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