PHPMailer is a core class in WordPress for sending emails. By default, it uses the PHP mail() function, but it can be configured to use the SMTP server. And, GD Mail Queue plugin has SMTP services for sending emails through PHPMailer using SMTP. Basic service is for setting up a custom SMTP server.
You need to provide SMTP server host address, port and encryption. For most SMTP servers, you need to include authentication information in the form of username and password for that SMTP server.
It is recommended to use encrypted SMTP’s (TLS or SSL), and make sure to include port specific to selected encryption. For information about that, make sure yo check your SMTP server documentation.