How do I send emails from a serverless function?

There are a few common ways to send email from a serverless function, and the best one depends on whether you want to send transactional email (password resets, receipts, alerts) or just need a simple SMTP setup.

Recommended approach: use an email API

For serverless functions, the easiest and most reliable method is usually to call a third-party email service API such as:

• SendGrid
• Mailgun
• Postmark
• Amazon SES
• Resend

Why this is best for serverless

• No need to manage your own mail server
• Works well with short-lived functions
• Better deliverability than raw SMTP in many cases
• Easier to configure with environment variables
• Supports retries, templates, logs, and webhooks

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Basic pattern

Your serverless function:

1. Receives a request
2. Validates the input
3. Calls the email provider’s API
4. Returns success/failure

Example: Node.js using SendGrid

import sgMail from '@sendgrid/mail';

sgMail.setApiKey(process.env.SENDGRID_API_KEY);

export default async function handler(req, res) {
  try {
    const { to, subject, text } = req.body;

    await sgMail.send({
      to,
      from: 'you@yourdomain.com',
      subject,
      text,
    });

    res.status(200).json({ message: 'Email sent' });
  } catch (error) {
    console.error(error);
    res.status(500).json({ error: 'Failed to send email' });
  }
}

Set your secret in environment variables:

• SENDGRID_API_KEY

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Alternative: SMTP

You can also send mail with SMTP using libraries like nodemailer.

Example: Node.js with Nodemailer

import nodemailer from 'nodemailer';

export default async function handler(req, res) {
  const transporter = nodemailer.createTransport({
    host: process.env.SMTP_HOST,
    port: 587,
    secure: false,
    auth: {
      user: process.env.SMTP_USER,
      pass: process.env.SMTP_PASS,
    },
  });

  try {
    await transporter.sendMail({
      from: 'you@yourdomain.com',
      to: req.body.to,
      subject: req.body.subject,
      text: req.body.text,
    });

    res.status(200).json({ message: 'Email sent' });
  } catch (err) {
    console.error(err);
    res.status(500).json({ error: 'Email failed' });
  }
}

Caveats with SMTP in serverless

• Cold starts can slow connection setup
• Some providers block SMTP or require extra configuration
• Connection reuse is limited in serverless environments
• More likely to hit timeouts if the SMTP server is slow

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Best practices

• Store credentials in environment variables, not code
• Verify sender domain with SPF/DKIM/DMARC
• Validate recipient addresses and input
• Use rate limiting to prevent abuse
• Keep function execution short
• Log provider errors for debugging
• Use templates for consistent formatting

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If you’re using a specific platform

• AWS Lambda: SES is a strong choice
• Vercel / Netlify / Cloudflare Workers: API-based providers are usually easiest
• Firebase Functions: SendGrid, Mailgun, or SES all work well

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If you want, I can give you a copy-paste example for your specific serverless platform (AWS Lambda, Vercel, Netlify, Cloudflare Workers, etc.) and language.