Transactional email

What it is

A transactional email service is an API for sending individual messages triggered by something a user did. It handles delivery, bounces and domain authentication.

How it works

1. Verify your sending domain with the authentication records the service gives you.
2. Call the API from your app, an automation or an agent to send a message from a template.
3. Monitor bounces and complaints. Resend documents an MCP server for agent access.

Plans and rules (checked 2026-10-17). Resend sells Free, Pro, Scale and Enterprise plans, priced by monthly email volume. Its API rate limit is the same on every plan, and batch requests send up to 100 emails at once. Google requires every sender to Gmail to authenticate with SPF or DKIM and to keep reported spam below 0.3%. Under the US CAN-SPAM Act, the FTC says purely transactional or relationship messages are exempt from most provisions. Once promotion becomes a message’s primary purpose, the full commercial-email rules apply.

Benefits

• Reliable, fast delivery of messages customers expect.
• Developer-friendly APIs fit automations and agents.
• Keeps transactional mail separate from marketing reputation.

Limits and risks

• Don’t mix in marketing. The FTC treats a message as commercial if its subject line or opening is promotional, which brings opt-out and other requirements.
• Limit agent sending. Agents that can send email need strict limits and logs.
• Inbox rules still apply. Inbox providers’ sender requirements cover transactional mail too.
• Plans change. This category page re-checks every 90 days, and each tool page carries its own review date.

This page summarizes published rules for orientation. It isn’t legal advice; check the rules that apply where you and your recipients are.