Can I Send Emails in Go Without Using an SMTP Server?

Sending Emails Without an SMTP Server in Go

Question:

You want to send bulk emails without relying on an external SMTP server due to usage limitations. Can you send emails without an SMTP server using Go's standard library?

Answer:

To send emails without an SMTP server, you must delegate the task to another program capable of sending emails. In POSIX systems, the /usr/sbin/sendmail program is often available for this purpose. You can call it with the -t option to retrieve recipient addresses from the message's To headers.

Using the Standard Library with Sendmail

You can use the os/exec, net/mail, and net/textproto packages to call Sendmail directly, but the gomail package provides a simpler solution. Its Message type includes a WriteTo() method that can write to a running Sendmail instance. Here's an example:

<code class="go">import (
    "os/exec"

    "github.com/go-gomail/gomail"
)

const sendmailPath = "/usr/sbin/sendmail"

func sendEmail(m *gomail.Message) error {
    cmd := exec.Command(sendmailPath, "-t")
    cmd.Stdout = os.Stdout
    cmd.Stderr = os.Stderr

    stdin, err := cmd.StdinPipe()
    if err != nil {
        return err
    }

    if err := cmd.Start(); err != nil {
        return err
    }

    if _, err := m.WriteTo(stdin); err != nil {
        return err
    }

    if err := stdin.Close(); err != nil {
        return err
    }

    if err := cmd.Wait(); err != nil {
        return err
    }

    return nil
}</code>

Advantages of Using an MTA:

While using an MTA may seem like an extra step, it offers the advantage of mail queuing. If an MTA cannot deliver a message immediately, such as during a network outage, it will store the message in a queue and repeatedly attempt to deliver it until successful or a timeout occurs.

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