SMTP

Go SMTP

Use Go's SMTP packages to send email through the Lettermint SMTP relay.

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

Basic configuration

Use environment variables for credentials:

Code
import "os"

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

var APIToken = os.Getenv("LETTERMINT_PROJECT_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_PROJECT_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:

Code
func sendEmailMultipleRecipients(to, cc, bcc []string, subject, body string) error {
    from := "sender@yourdomain.com"
    apiToken := os.Getenv("LETTERMINT_PROJECT_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:

Code
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_PROJECT_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:

Code
func sendEmailWithHeaders(to, subject, body string) error {
    from := "sender@yourdomain.com"
    apiToken := os.Getenv("LETTERMINT_PROJECT_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-Tags: campaign=orders, environment=production\r\n"+
        "X-LM-Tags: variant=B\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,
    )
}

X-LM-Tag is the singular tag. X-LM-Tags contains structured pairs. A raw message can contain more than one X-LM-Tags field.

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:

Code
go get gopkg.in/gomail.v2

Code
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_PROJECT_TOKEN"))

    return d.DialAndSend(m)
}

Python (smtplib)Ruby (Mail)