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This guide covers SMTP integration. For full configuration options and available headers, see the SMTP Introduction.

​
Basic Configuration

Use environment variables for credentials: 
import "os"

const (
    SMTPHost = "smtp.lettermint.co"
    Username = "lettermint"
)

var APIToken = os.Getenv("LETTERMINT_API_TOKEN")

package main

import (
    "fmt"
    "net/smtp"
    "os"
)

const (
    SMTPHost = "smtp.lettermint.co"
    SMTPPort = "587"
    Username = "lettermint"
)

func sendEmail(to, subject, body string) error {
    from := "sender@yourdomain.com"
    apiToken := os.Getenv("LETTERMINT_API_TOKEN")

    msg := []byte(fmt.Sprintf(
        "From: %s\r\n"+
        "To: %s\r\n"+
        "Subject: %s\r\n"+
        "Content-Type: text/html; charset=UTF-8\r\n"+
        "\r\n"+
        "%s\r\n",
        from, to, subject, body,
    ))

    auth := smtp.PlainAuth("", Username, apiToken, SMTPHost)

    return smtp.SendMail(
        SMTPHost+":"+SMTPPort,
        auth,
        from,
        []string{to},
        msg,
    )
}

func main() {
    err := sendEmail(
        "recipient@example.com",
        "Test Email",
        "<h1>Hello!</h1><p>This is a test email.</p>",
    )

    if err != nil {
        fmt.Printf("Error: %v\n", err)
    } else {
        fmt.Println("Email sent successfully")
    }
}

​
Advanced Features

​
Multiple Recipients

Send to multiple recipients with CC and BCC: 
func sendEmailMultipleRecipients(to, cc, bcc []string, subject, body string) error {
    from := "sender@yourdomain.com"
    apiToken := os.Getenv("LETTERMINT_API_TOKEN")

    // Build recipient list for headers
    toHeader := strings.Join(to, ", ")
    ccHeader := strings.Join(cc, ", ")

    headers := fmt.Sprintf(
        "From: %s\r\n"+
        "To: %s\r\n"+
        "Cc: %s\r\n"+
        "Subject: %s\r\n"+
        "Content-Type: text/html; charset=UTF-8\r\n"+
        "\r\n",
        from, toHeader, ccHeader, subject,
    )

    msg := []byte(headers + body + "\r\n")

    auth := smtp.PlainAuth("", Username, apiToken, SMTPHost)

    // All recipients (to + cc + bcc) receive the email
    allRecipients := append(append(to, cc...), bcc...)

    return smtp.SendMail(
        SMTPHost+":587",
        auth,
        from,
        allRecipients,
        msg,
    )
}

// Usage
sendEmailMultipleRecipients(
    []string{"user1@example.com", "user2@example.com"},
    []string{"manager@yourdomain.com"},
    []string{"archive@yourdomain.com"},
    "Monthly Newsletter",
    "<h1>This Month's Updates</h1>",
)

​
Attachments

Send emails with file attachments: 
import (
    "encoding/base64"
    "fmt"
    "io/ioutil"
    "mime"
    "net/smtp"
    "os"
    "path/filepath"
    "strings"
)

func sendEmailWithAttachment(to, subject, body, filePath string) error {
    from := "sender@yourdomain.com"
    apiToken := os.Getenv("LETTERMINT_API_TOKEN")

    // Read file
    fileContent, err := ioutil.ReadFile(filePath)
    if err != nil {
        return err
    }

    fileName := filepath.Base(filePath)
    mimeType := mime.TypeByExtension(filepath.Ext(filePath))
    if mimeType == "" {
        mimeType = "application/octet-stream"
    }

    boundary := "boundary123"

    var msg strings.Builder
    msg.WriteString(fmt.Sprintf("From: %s\r\n", from))
    msg.WriteString(fmt.Sprintf("To: %s\r\n", to))
    msg.WriteString(fmt.Sprintf("Subject: %s\r\n", subject))
    msg.WriteString(fmt.Sprintf("MIME-Version: 1.0\r\n"))
    msg.WriteString(fmt.Sprintf("Content-Type: multipart/mixed; boundary=%s\r\n", boundary))
    msg.WriteString("\r\n")

    // HTML body
    msg.WriteString(fmt.Sprintf("--%s\r\n", boundary))
    msg.WriteString("Content-Type: text/html; charset=UTF-8\r\n")
    msg.WriteString("\r\n")
    msg.WriteString(body)
    msg.WriteString("\r\n")

    // Attachment
    msg.WriteString(fmt.Sprintf("--%s\r\n", boundary))
    msg.WriteString(fmt.Sprintf("Content-Type: %s\r\n", mimeType))
    msg.WriteString("Content-Transfer-Encoding: base64\r\n")
    msg.WriteString(fmt.Sprintf("Content-Disposition: attachment; filename=\"%s\"\r\n", fileName))
    msg.WriteString("\r\n")
    msg.WriteString(base64.StdEncoding.EncodeToString(fileContent))
    msg.WriteString("\r\n")

    msg.WriteString(fmt.Sprintf("--%s--\r\n", boundary))

    auth := smtp.PlainAuth("", Username, apiToken, SMTPHost)

    return smtp.SendMail(
        SMTPHost+":587",
        auth,
        from,
        []string{to},
        []byte(msg.String()),
    )
}

​
Lettermint Headers

Add Lettermint-specific headers for tags, metadata, and routing: 
func sendEmailWithHeaders(to, subject, body string) error {
    from := "sender@yourdomain.com"
    apiToken := os.Getenv("LETTERMINT_API_TOKEN")

    msg := []byte(fmt.Sprintf(
        "From: %s\r\n"+
        "To: %s\r\n"+
        "Subject: %s\r\n"+
        "Content-Type: text/html; charset=UTF-8\r\n"+
        "X-LM-Tag: order-confirmation\r\n"+
        "X-LM-Metadata-order_id: 12345\r\n"+
        "X-LM-Metadata-customer_id: cust_789\r\n"+
        "X-Lettermint-Route: transactional\r\n"+
        "\r\n"+
        "%s\r\n",
        from, to, subject, body,
    ))

    auth := smtp.PlainAuth("", Username, apiToken, SMTPHost)

    return smtp.SendMail(
        SMTPHost+":587",
        auth,
        from,
        []string{to},
        msg,
    )
}
Metadata headers are extracted by Lettermint and included in webhook payloads, but not added to the actual email sent to recipients. See the SMTP Introduction for full details on available headers.

​
Using Third-Party Libraries

​
With gomail

For a simpler API, use the gomail library: 
go get gopkg.in/gomail.v2

package main

import (
    "os"
    "gopkg.in/gomail.v2"
)

func sendEmailWithGomail() error {
    m := gomail.NewMessage()
    m.SetHeader("From", "sender@yourdomain.com")
    m.SetHeader("To", "recipient@example.com")
    m.SetHeader("Subject", "Test Email")
    m.SetBody("text/html", "<h1>Hello!</h1><p>This is a test email.</p>")

    // Add Lettermint headers
    m.SetHeader("X-LM-Tag", "test-email")
    m.SetHeader("X-LM-Metadata-campaign", "test")

    // Add attachments
    m.Attach("/path/to/document.pdf")

    d := gomail.NewDialer("smtp.lettermint.co", 587, "lettermint", os.Getenv("LETTERMINT_API_TOKEN"))

    return d.DialAndSend(m)
}