I'm trying to use mailgun as my email service now. I did follow the steps (http://fuelphp.com/docs/packages/email/usage.html#/mailgun) on my local and it works, I was able to send email and I check my mailgun account and it was there telling me that I was able to send an email.
When uploading it on my real server. I uploaded the following. vendor/mailgun vendor/symfony vendor/guzzle app/config/email.php (email config for mailgun)
But I got this error when I'm trying to send an email.
Fatal Error - Class 'Mailgun\Mailgun' not found in /var/www/mysite/fuel/packages/email/classes/email/driver/mailgun.php on line 25
Looks like your missing a lot of composer stuff, in particular vendor/composer, which contains the composer autoloader and the autoloader configuration.
Since you are getting no error that the composer autoloader is missing, my guess is that you have an old version of vendor/composer on your server, that doesn't have the namespace for Mailgun registered.
Not sure, we only use dedicated servers and deploy using git (so composer runs on the servers).
I suggest you do it and see if it helps. When you run a composer install or update it will generate the autoloader files in vendor/composer with the namespaces of the packages hardcoded in it, so you have to make sure your vendor folder is always transferred completely.