What is the use of optOutPolicy while sending email using apex through SingleEmailMessage()?

Hi,

To Make it more simple to understand-

If Opt Out Policy is set in single email then no matter what your recipients will always receive the mail.

Messaging.SingleEmailMessage message = new Messaging.SingleEmailMessage();
message.toAddresses = new String[] { '[email protected]', '[email protected]' };
message.optOutPolicy = 'FILTER';
message.subject = 'Subject Test Message';
message.plainTextBody = 'This is the message body.';
Messaging.SingleEmailMessage[] messages = new List<Messaging.SingleEmailMessage> {message};
Messaging.SendEmailResult[] results = Messaging.sendEmail(messages);

if (results[0].success) {
System.debug('The email was sent successfully.');
} else {
System.debug('The email failed to send: ' + results[0].errors[0].message);
}

Hope this helps.