GD Mail Queue Pro plugin has support for custom SMTP services for PHPMailer. One of the services included is for the SendGrid email sending service. And, to start using this service to send emails using SMTP you need SendGrid account. After you have set up your SendGrid account, you need to get SMTP API Key to add to the GD Mail Queue Pro plugin SendGrid Service settings panel.
To get the required information, open your SendGrid account, open Settings, then open API Keys. Click Create API Key button, select Full Access and give the key a name, and click Create and View. Make sure to copy the key to some safe place, because you will not be able to see the key again. If you lose the key, you need to generate a new one. Insert copied key into the plugin settings page.
The plugin also allows you to set one or more categories to classify the emails coming from your website (you can use website name for instance) if you use more one account for more than one website.