can you try and add this in your local.config.php ?
Config::Set('EMAIL_USE_SMTP', true);
Config::Set('EMAIL_SMTP_USER', 'MAILGUN USERNAME');
Config::Set('EMAIL_SMTP_PASS', 'MAILGUN PASSWORD');
Config::Set('EMAIL_SMTP_SECURE', 'ssl');
Config::Set('EMAIL_SMTP_PORT', '465');
Comment out the previus settings