smtpmailer

package
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 Go to latest Published: Dec 21, 2025 License: MIT Imports: 4 Imported by: 0

Documentation

Overview

Package smtpmailer provides a simple SMTP email client with TLS support for sending HTML and plain text emails.

Basic Usage

Create a mailer and send an email: 

import smtpmailer "github.com/poly-workshop/go-webmods/smtpmailer"

mailer := smtpmailer.NewMailer(smtpmailer.Config{
    Host:      "smtp.gmail.com",
    Port:      587,
    Username:  "your-email@gmail.com",
    Password:  "your-app-password",
    FromEmail: "your-email@gmail.com",
    FromName:  "My Application",
})

err := mailer.SendEmail(smtpmailer.Message{
    ToEmails: []string{"user@example.com"},
    Subject:  "Welcome to our service",
    Body:     "Thank you for signing up!",
    IsHTML:   false,
})

HTML Emails

Send HTML-formatted emails: 

err := mailer.SendEmail(smtpmailer.Message{
    ToEmails: []string{"user@example.com"},
    Subject:  "Welcome!",
    Body: `
        <html>
            <body>
                <h1>Welcome to our service!</h1>
                <p>Thank you for signing up.</p>
                <a href="https://example.com/verify">Verify Email</a>
            </body>
        </html>
    `,
    IsHTML: true,
})

Multiple Recipients

Send to multiple recipients: 

err := mailer.SendEmail(smtpmailer.Message{
    ToEmails: []string{
        "user1@example.com",
        "user2@example.com",
        "user3@example.com",
    },
    Subject: "Newsletter",
    Body:    "Check out our latest updates!",
})

Common SMTP Providers

Gmail (requires App Password): 

Config{
    Host:     "smtp.gmail.com",
    Port:     587,
    Username: "your-email@gmail.com",
    Password: "app-password",  // Not your regular password
}

Office 365: 

Config{
    Host:     "smtp.office365.com",
    Port:     587,
    Username: "your-email@outlook.com",
    Password: "your-password",
}

SendGrid: 

Config{
    Host:     "smtp.sendgrid.net",
    Port:     587,
    Username: "apikey",
    Password: "your-sendgrid-api-key",
}

Mailgun: 

Config{
    Host:     "smtp.mailgun.org",
    Port:     587,
    Username: "postmaster@your-domain.mailgun.org",
    Password: "your-mailgun-password",
}

Configuration with Viper

Example configuration file (configs/default.yaml): 

smtp:
  host: smtp.gmail.com
  port: 587
  username: ${SMTP_USERNAME}
  password: ${SMTP_PASSWORD}
  from_email: noreply@example.com
  from_name: My Application

Loading configuration: 

import (
    "github.com/poly-workshop/go-webmods/app"
    "github.com/poly-workshop/go-webmods/smtpmailer"
)

app.Init(".")
cfg := app.Config()

mailer := smtpmailer.NewMailer(smtpmailer.Config{
    Host:      cfg.GetString("smtp.host"),
    Port:      cfg.GetInt("smtp.port"),
    Username:  cfg.GetString("smtp.username"),
    Password:  cfg.GetString("smtp.password"),
    FromEmail: cfg.GetString("smtp.from_email"),
    FromName:  cfg.GetString("smtp.from_name"),
})

Security

The mailer uses TLS for secure communication:

  • Connects via plain TCP initially
  • Upgrades to TLS using STARTTLS
  • Enforces minimum TLS version 1.2
  • Validates server certificate

Email Templates

For HTML emails with dynamic content, use Go's html/template: 

import (
    "bytes"
    "html/template"
)

tmpl, err := template.ParseFiles("templates/welcome.html")
if err != nil {
    return err
}

var body bytes.Buffer
err = tmpl.Execute(&body, struct {
    Name string
    URL  string
}{
    Name: "Alice",
    URL:  "https://example.com/verify?token=abc123",
})
if err != nil {
    return err
}

err = mailer.SendEmail(smtpmailer.Message{
    ToEmails: []string{"alice@example.com"},
    Subject:  "Verify your email",
    Body:     body.String(),
    IsHTML:   true,
})

Error Handling

SendEmail returns an error if:

  • Connection to SMTP server fails
  • TLS upgrade fails
  • Authentication fails
  • Email is rejected by the server
  • Network errors occur during sending

Always check the error: 

err := mailer.SendEmail(msg)
if err != nil {
    log.Printf("Failed to send email: %v", err)
    // Handle error (retry, log, notify admin, etc.)
    return err
}

Best Practices

  • Store SMTP credentials in environment variables or secret management
  • Use app passwords for Gmail (not your account password)
  • Set appropriate From name for better deliverability
  • Validate email addresses before sending
  • Use HTML templates for consistent styling
  • Include plain text alternatives for HTML emails (not currently supported, consider adding)
  • Implement retry logic for transient failures
  • Rate limit email sending to avoid being flagged as spam
  • Use dedicated email service providers (SendGrid, Mailgun) for production
  • Test emails thoroughly before production deployment

Rate Limiting

Implement rate limiting for bulk emails: 

import "time"

for _, recipient := range recipients {
    err := mailer.SendEmail(smtpmailer.Message{
        ToEmails: []string{recipient},
        Subject:  "Newsletter",
        Body:     content,
    })
    if err != nil {
        log.Printf("Failed to send to %s: %v", recipient, err)
    }
    time.Sleep(100 * time.Millisecond)  // Rate limit
}

Limitations

Current limitations:

  • No attachment support
  • No CC/BCC support
  • No plain text alternative for HTML emails
  • No email validation
  • No retry mechanism
  • No bulk sending optimization

For advanced features, consider using a dedicated email library or service.

Example: Password Reset Email 

func sendPasswordReset(mailer *smtpmailer.Mailer, email, token string) error {
    resetURL := fmt.Sprintf("https://example.com/reset?token=%s", token)

    body := fmt.Sprintf(`
        <html>
            <body style="font-family: Arial, sans-serif;">
                <h2>Password Reset Request</h2>
                <p>Click the link below to reset your password:</p>
                <a href="%s" style="display: inline-block; padding: 10px 20px; background-color: #007bff; color: white; text-decoration: none; border-radius: 5px;">
                    Reset Password
                </a>
                <p>This link will expire in 1 hour.</p>
                <p>If you didn't request this, please ignore this email.</p>
            </body>
        </html>
    `, resetURL)

    return mailer.SendEmail(smtpmailer.Message{
        ToEmails: []string{email},
        Subject:  "Password Reset Request",
        Body:     body,
        IsHTML:   true,
    })
}
Example

Example demonstrates basic email sending. 

package main

import (
	"fmt"
)

func main() {
	// mailer := smtpmailer.NewMailer(smtpmailer.Config{
	// 	Host:      "smtp.example.com",
	// 	Port:      587,
	// 	Username:  "user@example.com",
	// 	Password:  "password",
	// 	FromEmail: "noreply@example.com",
	// 	FromName:  "My Application",
	// })
	//
	// err := mailer.SendEmail(smtpmailer.Message{
	// 	ToEmails: []string{"recipient@example.com"},
	// 	Subject:  "Hello",
	// 	Body:     "This is a test email.",
	// 	IsHTML:   false,
	// })
	//
	// if err != nil {
	// 	fmt.Println("Error:", err)
	// 	return
	// }

	fmt.Println("Email sent successfully")
}

Output:
Email sent successfully
Example (Gmail)

Example_gmail demonstrates configuring Gmail SMTP. 

package main

import (
	"fmt"

	smtpmailer "github.com/poly-workshop/go-webmods/smtpmailer"
)

func main() {
	// Note: For Gmail, you need to use an App Password
	// Go to Google Account > Security > 2-Step Verification > App Passwords

	mailer := smtpmailer.NewMailer(smtpmailer.Config{
		Host:      "smtp.gmail.com",
		Port:      587,
		Username:  "your-email@gmail.com",
		Password:  "your-app-password", // Not your regular password!
		FromEmail: "your-email@gmail.com",
		FromName:  "My App",
	})

	_ = mailer
	fmt.Println("Gmail mailer configured")
}

Output:
Gmail mailer configured
Example (Html)

Example_html demonstrates sending HTML emails. 

package main

import (
	"fmt"
)

func main() {
	// mailer := smtpmailer.NewMailer(smtpmailer.Config{
	// 	Host:      "smtp.example.com",
	// 	Port:      587,
	// 	Username:  "user@example.com",
	// 	Password:  "password",
	// 	FromEmail: "noreply@example.com",
	// 	FromName:  "My Application",
	// })
	//
	// htmlBody := `
	// 	<html>
	// 		<body>
	// 			<h1>Welcome!</h1>
	// 			<p>Thank you for signing up.</p>
	// 			<a href="https://example.com/verify">Verify Email</a>
	// 		</body>
	// 	</html>
	// `
	//
	// err := mailer.SendEmail(smtpmailer.Message{
	// 	ToEmails: []string{"user@example.com"},
	// 	Subject:  "Welcome to Our Service",
	// 	Body:     htmlBody,
	// 	IsHTML:   true,
	// })
	//
	// if err != nil {
	// 	fmt.Println("Error:", err)
	// 	return
	// }

	fmt.Println("HTML email sent")
}

Output:
HTML email sent
Example (MultipleRecipients)

Example_multipleRecipients demonstrates sending to multiple recipients. 

package main

import (
	"fmt"
)

func main() {
	// mailer := smtpmailer.NewMailer(smtpmailer.Config{
	// 	Host:      "smtp.example.com",
	// 	Port:      587,
	// 	Username:  "user@example.com",
	// 	Password:  "password",
	// 	FromEmail: "noreply@example.com",
	// 	FromName:  "Newsletter",
	// })
	//
	// err := mailer.SendEmail(smtpmailer.Message{
	// 	ToEmails: []string{
	// 		"user1@example.com",
	// 		"user2@example.com",
	// 		"user3@example.com",
	// 	},
	// 	Subject: "Monthly Newsletter",
	// 	Body:    "Check out our latest updates!",
	// 	IsHTML:  false,
	// })
	//
	// if err != nil {
	// 	fmt.Println("Error:", err)
	// 	return
	// }

	fmt.Println("Newsletter sent to multiple recipients")
}

Output:
Newsletter sent to multiple recipients
Example (PasswordReset)

Example_passwordReset demonstrates a password reset email. 

package main

import (
	"fmt"
)

func main() {
	// mailer := smtpmailer.NewMailer(smtpmailer.Config{
	// 	Host:      "smtp.example.com",
	// 	Port:      587,
	// 	Username:  "user@example.com",
	// 	Password:  "password",
	// 	FromEmail: "noreply@example.com",
	// 	FromName:  "My Application",
	// })
	//
	// resetToken := "abc123xyz"
	// resetURL := fmt.Sprintf("https://example.com/reset?token=%s", resetToken)
	//
	// htmlBody := fmt.Sprintf(`
	// 	<html>
	// 		<body style="font-family: Arial, sans-serif;">
	// 			<h2>Password Reset Request</h2>
	// 			<p>Click the link below to reset your password:</p>
	// 			<a href="%s" style="display: inline-block; padding: 10px 20px; background-color: #007bff; color: white; text-decoration: none; border-radius: 5px;">
	// 				Reset Password
	// 			</a>
	// 			<p>This link will expire in 1 hour.</p>
	// 			<p>If you didn't request this, please ignore this email.</p>
	// 		</body>
	// 	</html>
	// `, resetURL)
	//
	// err := mailer.SendEmail(smtpmailer.Message{
	// 	ToEmails: []string{"user@example.com"},
	// 	Subject:  "Password Reset Request",
	// 	Body:     htmlBody,
	// 	IsHTML:   true,
	// })
	//
	// if err != nil {
	// 	fmt.Println("Error:", err)
	// 	return
	// }

	fmt.Println("Password reset email sent")
}

Output:
Password reset email sent

Index

Examples

Constants

This section is empty.

Variables

This section is empty.

Functions

This section is empty.

Types

type Config 

type Config struct {
	Host      string
	Port      int
	Username  string
	Password  string
	FromEmail string
	FromName  string
}

type Mailer 

type Mailer struct {
	// contains filtered or unexported fields
}

func NewMailer 

func NewMailer(config Config) *Mailer

func (*Mailer) SendEmail 

func (m *Mailer) SendEmail(msg Message) error

type Message 

type Message struct {
	ToEmails []string
	Subject  string
	Body     string
	IsHTML   bool
}

Source Files

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