Couple Suggestions

Good day! Just installed this plugin on a server that doesn’t have cURL enabled and noticed a couple of things.

1. Your plugin crashes as there’s no check for curl before curl_init() is called on line 17 of class-sendgrid-tools.php
2. Check out the wp_remote_get() function. I don’t have time to re-write and test all the cURL requests right now, but I tested with the first request you have and this function seems to be a viable substitute that should work on any WP site, regardless of whether cURL is installed or not: http://codex.wordpress.org/Function_Reference/wp_remote_get

Just a couple of tips! I discovered the curl issue on a plugin I wrote and have been looking into switching to the wp_remote_get function for this exact reason.

Have a great day!

https://wordpress.org/plugins/sendgrid-email-delivery-simplified/