Questions on Email Validation setting

Hi, I could not find the docs related to this Email Validation function for Post SMTP plugin. So by default the plugin enabled the Email Validation, so may I know what sort of validation process that included in this plugin?

To be exact, what are the checks that run before email is being sent? Does it just check the formatting only or include mailbox existence check?

Also what will happen if the checks/validation failed? Will this interrupt the other process chain (such as woocommerce order completion)?