I have written a PHP script which retrieves my customers from MySQL database (around 10,000 customers) and sends them an email. The problem is, it sends emails with around 5 emails per second. But I know that others are able to send with around 20 emails per second.
PHP script simply sends through local MTA with mail() function in every member loop. Example:
mail('email@example.com', 'subject', 'body', 'headers');
What can be done in sendmail configuration to speed up the delivery?