Lumen 7.x "Target class [mail.manager] does not exist."

Hello,

I recently upgraded to lumen 7.x and integrate the mailer. I followed the instruction in the docs (https://lumen.laravel.com/docs/7.x/mail) but I still have this issue:

"Illuminate\Contracts\Container\BindingResolutionException"
"Target class [mail.manager] does not exist."

Any tips how to fix this? Thanks in advance!