So, PHP will only allow you to declare a class with a unique name. It looks like the newsletter plugin is using the same class name as the SendGrid plugin, "SMTP".
This is not ideal, but it definitely happens, especially when plugins are built independently of each other.
I think the better option here is to uninstall SendGrid plugin and then use the Newsletter SendGrid extension - http://www.thenewsletterplugin.com/plugins/newsletter/sendgrid-module