I had the same problem and I've already tried everything and nothing seemed to work until I just changed the 'host' value in config.php to:
'host' => env('smtp.mailtrap.io'),
When I changed that it worked nicely, somehow it was using the default host " smtp.mailtrap.org" and ignoring the .env variable I was setting.
After making some test I realize that if I placed the env variable in this order it would worked as it shoulded:
MAIL_HOST=smtp.mailtrap.io
?MAIL_DRIVER=smtp
?MAIL_PORT=2525?
MAIL_USERNAME=xxxx
?MAIL_PASSWORD=xxx
?MAIL_ENCRYPTION=null