When sending emails using PHP's PEAR Mail package, many developers encounter an issue where their emails show "helo=localhost" in the headers. This default configuration can cause deliverability issues as many mail servers perform reverse DNS lookups and verify that the HELO matches your domain's DNS records.

Proper HELO/EHLO configuration is crucial because:

• Spam filters often reject messages with mismatched HELO identifiers
• Many receiving servers perform forward-confirmed reverse DNS checks
• Professional email systems expect domain alignment between HELO and From addresses

For a domain "domain.com" with MX records at "mail.domain.com", your HELO should match one of these:

1. domain.com (most common and recommended)
2. mail.domain.com (if you want to be more specific)

Here's how to properly configure the HELO parameter in PEAR Mail:

require_once "Mail.php";

$params = array(
    'host' => 'mail.domain.com',
    'port' => 25,
    'auth' => true,
    'username' => 'user@domain.com',
    'password' => 'password',
    'localhost' => 'domain.com' // This sets the HELO/EHLO value
);

$mailer = Mail::factory('smtp', $params);

After implementation, check your email headers. They should now show:

Received: from domain.com ([12.34.56.78]) helo=domain.com

instead of the previous:

Received: from domain.com ([12.34.56.78]) helo=localhost

If you're still having problems:

• Ensure your server's reverse DNS (PTR record) matches your HELO
• Verify your SPF record includes the HELO domain
• Check for any firewall restrictions on outbound SMTP
• Test with different ports (25, 587, 465) if one is blocked

For more complex setups, you might need:

$params = array(
    'host' => 'mail.domain.com',
    'port' => 587,
    'localhost' => 'mailer1.domain.com',
    'persist' => true,
    'pipelining' => true,
    'timeout' => 30
);

When sending emails through SMTP, the HELO/EHLO command is your first handshake with the recipient's mail server. Many spam filters will check whether the domain you declare in HELO matches:

• Your MX records (mail.domain.com in your case)
• Your reverse DNS (PTR record)
• The domain in your From: address

Your current configuration shows:

Received: from domain.com ([12.34.56.78] helo=localhost)

This is problematic because:

1. localhost isn't a valid domain
2. It doesn't match your actual domain (domain.com)
3. Many spam filters will penalize or reject such emails

Here's how to correctly set the HELO parameter:

require_once "Mail.php";

$params = array(
    'host' => 'mail.domain.com',
    'auth' => true,
    'username' => 'user@domain.com',
    'password' => 'password',
    'localhost' => 'domain.com'  // This sets the HELO/EHLO value
);

$mail = Mail::factory('smtp', $params);

After implementation, check your email headers. They should now show:

Received: from domain.com ([12.34.56.78] helo=domain.com)

You can verify this using:

telnet mail.domain.com 25
EHLO domain.com

• Ensure your PTR record matches your HELO domain
• Set up SPF records for domain.com
• Consider implementing DKIM signing
• Test your configuration using tools like Mail-Tester.com

If you encounter problems:

// For debugging, add these parameters:
$params['debug'] = true;
$params['persist'] = false;

Common error messages and solutions:

1. "501 Invalid domain address" - Check your domain syntax
2. "550 Reverse DNS mismatch" - Verify your PTR records
3. "504 HELO requires domain address" - Ensure you're not using localhost