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Bounced email handling

Checking for bounced or undelivered emails in PHP requires a multi-step approach, as the built-in mail() function only confirms that the email was accepted by the local mail server, not its final delivery status. 
The reliable method involves using a dedicated bounce handling script that connects to a specific mailbox to retrieve and parse bounce messages sent back by remote mail servers. 

Method 1: Using a Bounce Handler Script
This approach involves setting up a dedicated email address (e.g., bounce@yourdomain.com) and then using a PHP script with a mail library to connect to that mailbox, read incoming messages, and identify bounce notifications. 
Steps:
  1. Configure a Return Path: When sending email, specify the Return-Path header to point all bounce messages to your dedicated bounce handling address.
    php
    $to = 'recipient@example.com';
$subject = 'Test Subject';
$message = 'Test Message';
$headers = 'From: webmaster@yourdomain.com' . "\r\n";
// Important: Set the return path for bounces
$return_path = '-fbounce@yourdomain.com'; 

// The mail function returns TRUE if accepted by the local MTA, not if delivered
if (mail($to, $subject, $message, $headers, $return_path)) {
    echo "Email accepted by local MTA, awaiting bounce notifications.\n";
} else {
    echo "Local server error.\n";
}
  2. Set up a Cron Job: Create a PHP script to periodically check the bounce@yourdomain.com inbox (e.g., every 15 minutes) using an email protocol like IMAP or POP3.
  3. Parse Bounce Messages: The script connects to the inbox and looks for specific keywords or headers (Final-Recipient, status: 5.X.X, Mailer-Daemon, etc.) within the incoming emails to identify the bounced address and the reason for the failure.
  4. Log and Act: Once the bounced email address is identified, your script can log it in a database and take appropriate action (e.g., unsubscribe the address from your mailing list to protect your sender reputation). 
A complete, robust solution often requires a third-party library to handle the complexities of parsing various bounce formats reliably. You can find examples of such libraries on GitHub or refer to advanced guides on sites like Stack Overflow. 

Method 2: Using a Third-Party Email Service
For a more reliable and less complex solution, many developers use third-party email services (like Postmark, Mailtrap, AWS SES, etc.). These services provide a dedicated API that gives explicit delivery statuses, including bounces, deferred, and opened events, removing the need for you to manage a bounce mailbox and parsing script. 
This is the recommended approach for production systems as it is more robust and scalable.

Basic Email Validation (Initial Check)
Before sending, you can use PHP's built-in functions to check if an email address is at least syntactically correct, which prevents immediate local errors. This does not, however, guarantee the mailbox actually exists. 

php
$email = "test@example.com";

if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
    echo "$email is a valid email format.\n";
} else {
    echo "$email is an invalid email format.\n";
}
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