You'll want to set MAIL_DRIVER=mailgun if you intend to use the MAILGUN_DOMAIN and MAILGUN_SECRET variables. You can also use SMTP with Mailgun instead of the API credentials since Mailgun supports both ways, but you currently have it set to use Mailtrap instead of Mailgun (Mailtrap != Mailgun).
Mailgun API and SMTP credentials: https://help.mailgun.com/hc/en-us/articles/203380100-Where-can-I-find-my-API-key-and-SMTP-credentials