Best Email API for Developers

Command references used in this guide: nylas email send and nylas email list.

What is an email API and why do developers use them?

An email API is a programmatic interface that lets applications send, receive, or manage email without configuring SMTP servers directly. According to Statista, over 360 billion emails were sent per day in 2024, and transactional messages (order confirmations, password resets, shipping updates) account for roughly 80% of commercial email volume. Developers use email APIs to offload deliverability, authentication, and scaling to a managed service.

There are two categories. Transactional email APIs (SendGrid, Mailgun, SES, Resend, Postmark) act as a managed SMTP relay: your app sends through their servers, and they handle SPF, DKIM, IP reputation, and bounce processing. Contextual email APIs (Nylas) connect to existing user mailboxes via OAuth and let your app read, search, send, and sync mail on behalf of the user. The category you need depends on whether you're sending from your domain or accessing someone else's inbox.

How do the 6 email APIs compare?

The table below compares all six APIs across 8 dimensions that matter when picking an email service. Pricing reflects public pricing pages as of May 2026. Free tiers vary from 100 emails/day (SendGrid) to 3,000 emails/month (SES, for accounts created before July 2025). Setup time assumes a fresh account with a custom sending domain and DNS verification complete.

At raw volume, Amazon SES is the cheapest: $0.10 per 1,000 emails, no monthly minimum. SendGrid and Resend both offer plans around $20/month for 50,000 emails. Postmark is the most expensive per-message but claims the highest inbox placement rate, advertising 99%+ delivery within 10 seconds. Nylas doesn't compete on transactional volume because it solves a different problem: accessing someone else's mailbox.

How do transactional email APIs work?

Transactional email APIs accept an API call with a recipient, subject, and body, then deliver the message through their own SMTP infrastructure. Your app never touches an SMTP server. The API provider manages IP warm-up, SPF/DKIM signing, bounce handling, and feedback loops with mailbox providers. SendGrid processes over 100 billion emails per month across its customer base, according to Twilio's Q4 2024 earnings report.

The workflow is the same across all five transactional APIs: sign up, verify your sending domain with DNS records (SPF, DKIM, DMARC), generate an API key, and start making HTTP POST requests. Domain verification typically takes 5-15 minutes. All five support template rendering, batch sends, and webhook-based delivery tracking. The key differences are pricing tiers, SDK quality, and deliverability reputation.

# SendGrid — send via API (Python)
import sendgrid
sg = sendgrid.SendGridAPIClient(api_key="SG.your-key")
message = sendgrid.helpers.mail.Mail(
    from_email="you@yourdomain.com",
    to_emails="recipient@example.com",
    subject="Order confirmation #4521",
    html_content="<p>Your order shipped.</p>"
)
sg.send(message)

# Amazon SES — send via AWS CLI
aws ses send-email \
  --from you@yourdomain.com \
  --destination ToAddresses=recipient@example.com \
  --message "Subject={Data='Order shipped'},Body={Text={Data='Your order shipped.'}}"

# Resend — send via API (curl)
curl -X POST https://api.resend.com/emails \
  -H "Authorization: Bearer re_your-key" \
  -H "Content-Type: application/json" \
  -d '{"from":"you@yourdomain.com","to":"recipient@example.com","subject":"Order shipped","html":"<p>Done.</p>"}'

All five services require you to own the sending domain. You can't send from user@gmail.com through SendGrid. That's the fundamental constraint of transactional email: it sends from your infrastructure, not from a user's inbox.

How do contextual email APIs work?

A contextual email API connects to an existing user's mailbox via OAuth and lets your application act on behalf of that user. Instead of sending from your domain, you're reading, searching, and sending from the user's own inbox. Nylas connects to Gmail, Outlook, Exchange, Yahoo, iCloud, and any IMAP provider through a single API. OAuth tokens refresh automatically every 3,600 seconds, so your app doesn't manage credentials.

The use cases are different from transactional email. CRM integrations read user email to log conversations. AI agents triage inboxes and draft replies. Scheduling tools read calendars and send invites from the user's address. These workflows can't run on SendGrid because they need inbox access, not just outbound delivery. The Nylas CLI exposes this same API through terminal commands, so you can test read and send operations without writing code.

# Install and authenticate
brew install nylas/nylas-cli/nylas
nylas init

# Read the 5 most recent emails from the connected mailbox
nylas email list --limit 5

# Send from the user's own address
nylas email send --to colleague@company.com \
  --subject "Meeting notes" \
  --body "Attached are the notes from today's standup."

The CLI's --json flag outputs structured data for scripting. Pipe it into jq to extract sender addresses, filter by date, or feed messages into an AI pipeline. Transactional APIs don't have an equivalent because they only handle outbound mail.

How do you choose the right email API?

The choice comes down to one question: are you sending from your own domain, or accessing a user's mailbox? If you're sending password resets, invoices, or marketing campaigns from noreply@yourdomain.com, use a transactional API. If your app reads, searches, or sends from the user's own email address, use a contextual API. Some apps need both. A SaaS product might send system notifications through Postmark and sync user email for a CRM feature through Nylas.

• Lowest cost at scale: Amazon SES at $0.10/1K emails. No monthly minimum, but requires AWS account and IAM setup. SES processed over 30 billion emails per month in 2024 per AWS re:Invent announcements.
• Fastest delivery: Postmark, which guarantees 99%+ delivery within 10 seconds for transactional streams. They don't allow marketing email on the same infrastructure.
• Best developer experience (send-only): Resend. Their REST API is clean, the React Email component library has 19,000+ GitHub stars, and pricing starts at $20/month for 50K emails.
• Read + send + calendar across providers: Nylas. One API covers Gmail, Outlook, Exchange, Yahoo, iCloud, and IMAP. OAuth-based, so users authenticate once and your app accesses their inbox programmatically.
• Most established: SendGrid, acquired by Twilio in 2019 for $3 billion. Largest customer base, most integrations, but the dashboard and documentation haven't aged well compared to newer entrants.

Don't pick an email API based on features you might need later. If you're building a notification system, start with the cheapest transactional option (SES or Resend). If you're building an inbox integration, start with a contextual API. Mixing both is normal. The SendGrid vs Nylas comparison goes deeper on the two most common APIs developers evaluate side by side.

Next steps

• SendGrid vs Nylas — head-to-head comparison of transactional vs contextual email
• Email APIs for AI agents — 14-criteria comparison for agent email access
• Gmail SMTP settings — ports, TLS, app passwords, and OAuth for direct SMTP
• Full command reference — every flag and subcommand documented