Sendmail in Laravel 5

I am new in Laravel 5. I tried to make sendmail feature for sign up email confirmation. I'm still in development stage, and I've set up my config/mail.php like many folks have said, and also the .env file too. I'm using mailgun provider. My question is, can i used the default sandbox domain mailgun in my config, or create custom domain, since I'm still in dev-stages. I got error like '..curl certificate..." after implemented those folks have said about the configuration. How to configure it using mailgun plus how to configure the mailgun account for domain we used in our laravel 5 web apps?. Thanks

via Chebli Mohamed