When troubleshooting undelivered emails, we need to understand the SMTP transaction flow. A complete delivery involves these stages:

1. Your SMTP server connects to recipient's MX server
2. Servers exchange HELO/EHLO commands
3. MAIL FROM and RCPT TO commands are executed
4. DATA transmission occurs
5. Transaction concludes with QUIT

Your mail server logs contain the most authoritative delivery evidence. Here's how to examine them:

# Postfix log example (typically /var/log/mail.log)
grep "unique-message-id" /var/log/mail.log

# Exim log search
exiqgrep -i message_id | exim -Mvl

Look for these key indicators:

• 250 2.0.0 Ok: Successful delivery
• 421 Service not available: Temporary failure
• 550 Mailbox unavailable: Permanent failure

The original email headers contain delivery metadata. For any sent message, you can extract headers with:

# Python example to extract headers
import email

with open('sent_email.eml', 'r') as f:
    msg = email.message_from_file(f)
    print(msg.items())  # Prints all headers

For direct verification without relying on logs, use these diagnostic tools:

# Telnet test to recipient's MX server
telnet mx.recipient.com 25
EHLO yourdomain.com
MAIL FROM: <sender@yourdomain.com>
RCPT TO: <recipient@theirdomain.com>

Common SMTP response codes to watch for:

For mission-critical emails, consider implementing these techniques:

# PHP implementation of Delivery Status Notification (DSN)
$mailer = new PHPMailer();
$mailer->DSN = true;
$mailer->Debugoutput = function($str, $level) {
    file_put_contents('smtp_debug.log', "$level: $str\n", FILE_APPEND);
};

Enterprise solutions include:

• SPF/DKIM/DMARC authentication logging
• Message tracking in Exchange Server
• Third-party delivery services (SendGrid, Mailgun)

When your mail server sends an email, the SMTP protocol provides several ways to track delivery status. The key components are:

• SMTP response codes (2xx, 4xx, 5xx series)
• Delivery Status Notifications (DSN)
• Message headers containing routing information

The first place to look is your mail server's logs. Here's a typical Postfix log entry for successful delivery:

Jan  1 12:34:56 mail postfix/smtp[1234]: ABC1234567: to=, relay=mx1.domain.com[1.2.3.4]:25, delay=1.2, delays=0.1/0.2/0.5/0.4, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as XYZ987)

You can manually verify if the recipient's server would accept your email:

telnet mx1.recipientdomain.com 25
HELO yourdomain.com
MAIL FROM: you@yourdomain.com
RCPT TO: recipient@recipientdomain.com
QUIT

Look for 250 response codes which indicate acceptance.

Request the recipient to provide full headers if they claim non-receipt. Key headers to examine:

• Received: (shows server hops)
• X-Received: (Gmail specific)
• Authentication-Results: (SPF/DKIM/DMARC)

For future emails, configure DSN requests in your mail server. Example for Postfix:

smtpd_discard_ehlo_keywords = 
smtpd_discard_ehlo_keywords_address_maps = 
default_destination_recipient_limit = 1

Here's a Python script to check delivery status using headers:

import email
import re

def check_delivery(headers):
    received_headers = [h for h in headers if h.startswith('Received')]
    last_server = received_headers[-1] if received_headers else None
    
    if last_server and 'recipientdomain.com' in last_server:
        return True
    return False

• Recipient server greylisting
• IP reputation issues
• Strict SPF/DKIM/DMARC policies
• Mailbox storage limits

For comprehensive tracking, consider implementing a commercial email tracking solution that provides read receipts and detailed analytics.