How to verify if an email inbox is full when sending emails using PHP?

In PHP, we can send emails using the SMTP protocol. However, the SMTP protocol itself cannot directly confirm if the mailbox is full. It can be indirectly determined through the following methods:

1. Error messages from the mailbox: When the SMTP server returns error messages, it may contain information about the mailbox being full. You can determine if the mailbox is full by capturing these error messages.

$to = 'recipient@example.com';
$subject = 'Test Email';
$message = 'This is a test email';

$headers = 'From: sender@example.com' . "\r\n" .
           'Reply-To: sender@example.com' . "\r\n" .
           'X-Mailer: PHP/' . phpversion();

if(mail($to, $subject, $message, $headers)){
    echo 'Email sent successfully.';
}else{
    echo 'Email could not be sent. Error: ' . error_get_last()['message'];
}

1. SMTP log: By enabling SMTP logging, you can review the communication details with the SMTP server. This will display all commands and the responses returned by the server. You can check the log for any error messages regarding a full mailbox.

ini_set('SMTP', 'smtp.example.com');
ini_set('smtp_port', 587);
ini_set('sendmail_from', 'sender@example.com');
ini_set('mail.log', 'smtp.log');

$to = 'recipient@example.com';
$subject = 'Test Email';
$message = 'This is a test email';

$headers = 'From: sender@example.com' . "\r\n" .
           'Reply-To: sender@example.com' . "\r\n" .
           'X-Mailer: PHP/' . phpversion();

if(mail($to, $subject, $message, $headers)){
    echo 'Email sent successfully.';
}else{
    echo 'Email could not be sent. Check the SMTP log for more details.';
}

In the example above, SMTP logs will be recorded in a file named smtp.log. You can open this file to view detailed information about SMTP communication, including any error messages related to mailbox fullness.

Please note that specific SMTP servers may return different error messages, so handling error messages may vary depending on the server. You may need to refer to the SMTP server you are using and related documents to determine how to parse the error messages.