SMTP Relay: What It Is, How It Works?

[Rajat K]

SMTP(Simple Mail Transfer Protocol) relay is a method of sending email through a third-party mail server (like SendGrid, Amazon SES, Mailgun, etc.) rather than your mail server.

Your server/app passes the email to the SMTP relay, which forwards (relays) it to the recipient.

When and why use SMTP relay?

• You're sending an email from an app bypassing the your server's default email sending limit.
• You want to avoid running your own mail server, often risky.
• Monitor and track opens, clicks, bounces, and more.

Platforms that provide SMTP relay services

• SendGrid
• Mailgun
• Amazon SES

How to set it up

1. Create an account on an SMTP relay provider (SendGrid, etc.)
2. Verify your domain.
3. Generate API Key (Remember API key is often displayed once, copy and keep it secured for future usage)
   • Under Settings > API Keys, create a key with "Full Access" or "Restricted Access" as needed
4. Configure SMTP Settings in Your App or Email Client
   • Use these settings
     SMTP server: smtp.sendgrid.net (for SendGrid)
     Port: 587 (TLS) or 465 (SSL)
     Username: apikey (literally the word “apikey”)
     Password: your generated API key
5. Authenticate your domain:
   1. Add SPF, DKIM, DMARC records to your domain DNS
6. Test sending
7. Warm up your IP and inbox before starting

Things to Avoid

• Exceeding Rate Limits – Use proper sending intervals.