Why Does My PHPMailer Code Fail to Connect to the SMTP Host, While My Email Client Works?

Overcoming PHPMailer's "SMTP Error: Could Not Connect to SMTP Host" Obstacle

PHPMailer, a popular PHP mail sending library, sometimes throws the error "SMTP Error: Could Not Connect to SMTP Host." This issue can be encountered despite successfully sending emails through email clients like Thunderbird.

To troubleshoot, examine the settings used in the working Thunderbird configuration and compare them to those employed by PHPMailer. The following table highlights the key differences:

Setting	Thunderbird	PHPMailer
Server Name	mail.exampleserver.com	SMTP_HOST
Port	587	SMTP_PORT
Username	[email protected]	SMTP_USER
Secure Authentication	No	SMTPAuth
Connection Security	STARTTLS	Not specified

The PHPMailer code provided:

$mail = new PHPMailer();
$mail->IsSMTP();
$mail->Host = SMTP_HOST;
$mail->Port = SMTP_PORT;
$mail->SMTPAuth = true;
$mail->Username = SMTP_USER;
$mail->Password = SMTP_PASSWORD;
$mail->From = MAIL_SYSTEM;
$mail->FromName = MAIL_SYSTEM_NAME;
$mail->AddAddress($aSecuredGetRequest['email']);
$mail->IsHTML(true);

While these settings appear similar to the working Thunderbird configuration, the key difference lies in the lack of connection security specification in the PHPMailer code. To resolve this issue, add the following line to your PHPMailer script:

$mail->SMTPSecure = 'STARTTLS';

This line specifies that the connection to the SMTP server should use STARTTLS encryption. Once this adjustment is made, your PHPMailer script should successfully send emails.

Note: If you experience an error related to certificate verification failure when using PHPMailer version 5.6 or later, you can implement the following workaround:

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

Remember that this workaround should be a temporary measure, and you should address the underlying certificate issue by replacing the invalid certificate with a valid one.

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