Why Does PHPMailer Throw a \"Peer Certificate Mismatch\" Error Under PHP 5.6?

PHPMailer Warning: Peer Certificate Mismatch

PHPMailer users encountering the "PHP Warning: stream_socket_enable_crypto(): Peer certificate did not match expected" error under PHP 5.6 may be facing issues with certificate validation.

Background:

PHP 5.6 introduced stricter certificate verification for SSL connections. As a result, if the remote server's SSL configuration is incorrect, PHPMailer will fail to encrypt the connection.

Symptoms:

• Connection failure error: PHPMailer displays "Could not connect to SMTP host."
• Logging error: The log shows a certificate mismatch warning: "Peer certificate CN=*.mail.dreamhost.com...' did not match expected CN=mx1.sub4.homie.mail.dreamhost.com...'".
• Immediate QUIT command: PHPMailer may issue a QUIT command after attempting STARTTLS, indicating an encryption failure.

Solution:

The recommended solution is to correct the certificate or verification settings on the remote server. This may involve replacing an invalid certificate with a valid one or reconfiguring the SSL settings.

Alternative:

If immediate delivery is necessary and the certificate mismatch is not critical, you can disable certificate validation in PHPMailer using the following options:

$mail->SMTPOptions = array (
    'ssl' => array(
        'verify_peer'  => false,
        'verify_peer_name'  => false,
        'allow_self_signed' => true));

Note: Disabling certificate validation is not recommended as it compromises security by potentially accepting invalid certificates.

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