Just upgraded from Laravel 6 to 7, and had the error response above when submitting a contact form. I eventually found a solution that seems to work and I am submitting here to help out the next guy.
In the terminal run:
composer require illuminate/mail
Add the following to the top of the controller file (app/Http/Controllers/Main.php in my case):
use \Illuminate\Support\Facades\Mail;
Add this to bootstrap/app.php:
$app->register(Illuminate\Mail\MailServiceProvider::class);
Save and test, and it worked localhost.
If the above does not work for you, there are some other possible issues and solutions available at this link, where I must give credit to vipindasks.
https://laracasts.com/discuss/channels/lumen/lumen-52-mail-not-working
Since I am suppose to ask a question:
Do you see any problems with this solution even though the site and mailer is working now?
You are simply missing a backslash. This tells the autoloader that the file you are looking for is not in the namespace your controller resides in :
$app->register(\Illuminate\Mail\MailServiceProvider::class);