Why Gmail and SendGrid Don't Work for AI Agents (And What Does)

What actually changes when AI agents start sending emails?

The email protocol itself might not change, but everything else does. Pricing models become confusing, managing inboxes gets harder, and tools made for people don’t work well with autonomous senders. This post breaks down where things fail and what to design for instead.

But what if there was a different solution? One designed specifically for agents?

If you’re new to agent email, start by learning why AI agents need email to understand the basics.

Why Gmail Doesn’t Work

The limitations:

• No programmatic inbox creation: Each inbox requires manual Google Workspace setup. You can’t spin up 100 inboxes via API. Why does this matter? Think about real agent architectures. You might have separate inboxes for different agent roles: one for outreach, one for support, one for notifications. Or you’re running parallel workflows where each execution needs to track its own email thread without cross-contamination. Or you need isolated environments for dev, staging, and production. Without programmatic creation, every new workflow or environment means bottlenecks in another manual setup. Threads from different contexts start colliding. You lose traceability. Gmail forces you through admin consoles for each inbox. That doesn’t scale.
• OAuth complexity: The authentication process expects a person to click consent buttons. If a token refresh fails in the middle of the night, you have to rush to fix it.
• Restrictive rate limits: Besides daily sending caps (500 for free accounts, 2,000 for Workspace), there are also per-minute limits through the API. If your agent hits these limits, requests fail, and messages pile up. This can cause important notifications to be delayed or missed. For agents that need to work in real time, this unpredictability is a big issue.

The cost problem:

Imagine your system runs hundreds of agents: customer support bots, sales assistants, scheduling coordinators, notification handlers. Each needs an inbox. At $12 per user per month for Google Workspace, that’s $1,200/month for 100 agents—$12,000/month for 1,000. The math stops making sense fast.

That’s why many agent builders use workarounds. They might use catch-all addresses that send everything to one inbox, rely on transactional APIs like SendGrid for sending only, or set up shared mailboxes with complicated filters. But each workaround brings its own set of problems.

The real cost isn’t just the subscription fee. It’s also the engineering time spent maintaining workarounds that weren’t meant for autonomous systems.

Why Transactional APIs Aren’t Enough

SendGrid, Mailgun, and Resend all support inbound email. SendGrid’s Inbound Parse and Mailgun’s Routes can POST incoming messages to your webhook endpoint. You get the raw email data: headers, body, attachments. Problem solved?

Not quite. These webhooks are stateless. Each POST is a disconnected event. The message arrives, your endpoint processes it, and then it’s gone. There’s no persistent store. No way to query previous messages. No threading that links replies to their original conversations.

Here’s what that means in practice. Your agent sends an email on Monday. A customer replies on Thursday. The webhook fires when the reply is sent, but your agent has no record of Monday’s message. The In-Reply-To and References headers point to a Message-ID your system never stored. To reconstruct context, you need to build your own storage layer, threading logic, and search index.

An agent is only as good as its context. With disconnected events, every interaction feels stateless. There’s no memory of what came before, no awareness of what should come next. You’re essentially rebuilding inbox infrastructure from scratch on top of transactional APIs.

This is the difference between receiving messages and having an inbox. An inbox provides:

• Persistent storage: Messages stay retrievable, not fire-and-forget
• Threading: Conversations linked by Message-ID, In-Reply-To, References
• Search: Query history by sender, subject, date, and content
• State: Know what’s read, unread, replied, pending

An agent can’t reason over what it can’t remember.

What to Look For

Native agent email infrastructure should solve these problems by design.

These features give agents what they really need: authentication, two-way communication, audit trails, and threaded conversations. For more details, see our first blog post on why AI agents need email.

1. Programmatic Inbox Creation

Create inboxes instantly using an API instead of admin panels. You can scale from 1 to 10,000 inboxes with no manual work or OAuth issues. One API call creates an inbox, and you can delete it when you’re finished.

2. Built for Two-Way Communication

Send and receive emails. A native event-driven system, such as real-time webhooks or WebSockets, notifies your agent when emails arrive. You can also get events for sent, bounced, complained, or rejected emails for more insights. Threading and conversation management help keep context across exchanges.

3. Easy Deliverability

If email authentication isn’t set up correctly, your messages will go to spam. Three protocols are important:

• SPF: DNS record listing which servers can send on your domain’s behalf
• DKIM: Digital signature proving the message wasn’t altered in transit
• DMARC: Policy instructing receivers how to handle authentication failures

Agent-ready systems set up all three protocols automatically. You don’t have to do manual DNS troubleshooting, and your IP reputation is managed for you.

Comparison:

The Bottom Line

Gmail charges for each inbox and needs manual setup every time. SendGrid is mainly designed for sending emails, and inbound parsing was added later as an extra feature.

The bigger issue is that agents are a new type of internet user. They don’t click buttons or wait for pages to load. They work programmatically, at scale, and in parallel.

Traditional email infrastructure was designed around human patterns. Login flows expect someone to be present, and pricing is based on one inbox per person. Agents don’t fit these assumptions.

Gmail and SendGrid are good tools. But they weren’t designed for autonomous systems. AgentMail is.