Explain how to send large amounts of emails using PHP.

There are different methods through which we can send mails in PHP. They are as follows:

1. PHP mail() function
   It implicitly sends a message to SMTP server, which is configured in the php.ini file. This function is used by the base class of MIME message composing and sending package.
2. SMTP server relay
   They are used to relay the messages to an intermediate SMTP server. This server stores the messages temporarily and will try to deliver them in the destination SMTP server.
3. Sending urgent messages by doing direct delivery to the destination SMTP server
   A variable named direct_delivery is provided by the smtp_message_class sub-class, which connects to the destination SMTP server and sends the message directly.