8 Best Free Transactional Email APIs for Go (2026), Tested

client := resend.NewClient(os.Getenv("RESEND_API_KEY"))
params := &resend.SendEmailRequest{
    From:    "you@yourdomain.com",
    To:      []string{"user@example.com"},
    Subject: "Hello",
    Html:    "<p>It works.</p>",
}
sent, err := client.Emails.Send(params)

cfg, err := config.LoadDefaultConfig(ctx)
client := sesv2.NewFromConfig(cfg)
_, err = client.SendEmail(ctx, &sesv2.SendEmailInput{
    FromEmailAddress: aws.String("you@yourdomain.com"),
    Destination:      &types.Destination{ToAddresses: []string{"user@example.com"}},
    Content: &types.EmailContent{Simple: &types.Message{
        Subject: &types.Content{Data: aws.String("Hello")},
        Body:    &types.Body{Text: &types.Content{Data: aws.String("It works.")}},
    }},
})

auth := smtp.PlainAuth("", os.Getenv("SMTP_USER"), os.Getenv("SMTP_PASS"), "mail.smtp2go.com")
msg := []byte("From: you@yourdomain.com\r\n" +
    "To: user@example.com\r\n" +
    "Subject: Hello\r\n\r\n" +
    "It works.\r\n")
err := smtp.SendMail("mail.smtp2go.com:587", auth,
    "you@yourdomain.com", []string{"user@example.com"}, msg)