Sending e-mail with Laravel error "Class Swift_Transport_Esmtp_AuthHandler does not exist"

I'm new Laravel 5 developer and I have an error with emails. I can't send any email through the webapp. I have tried to send an email for reset a password or receive a new user, but I can't do it. It appears the following error:

(1/1) ReflectionException
Class Swift_Transport_Esmtp_AuthHandler does not exist

in DependencyContainer.php (line 309)
at ReflectionClass->__construct('Swift_Transport_Esmtp_AuthHandler')
in DependencyContainer.php (line 309)
at Swift_DependencyContainer->_createNewInstance('transport.authhandler')
in DependencyContainer.php (line 112)...

The SMTP config in ".env" file is correct. I'm using Mailtrap:

MAIL_DRIVER=smtp
MAIL_HOST=smtp.mailtrap.io
MAIL_PORT=2525
MAIL_USERNAME=54743344954e5b
MAIL_PASSWORD=06b34d8095e637
MAIL_ENCRYPTION=null

I don't know what's going on...

Thanks in advance

via Chebli Mohamed