Following feature is meant for those customers who actually send Transactional emails and they don’t want to suppress their transactional emails to those people who have opted out from other emails like marketing related.
Now by enabling the following custom settings, you can send emails to the contacts or leads even though they have Email Opt Out set as TRUE. This feature works well when you are planning to send any Transactional emails but not any marketing emails.
Click on Setup on the top right corner of your screen.
In Quick Find, search for Custom Settings --> CustomSite_URL-->Manage Button
Click on Manage button -->Site_URL
Uncheck ‘Suppress Email Opt Out’ field or in other words, set that field to “false” and Save the record.
It will give the ability to send emails to any Email Opt Out Contact and/or Lead