Hi! There can be multiple reasons why it is not showing up. You can include various content in an email even potentionally harmful. Of course that is not your intention, yet mail clients usually don't load external sources without permission. The asset you provide is an external source (linked from your site) if it is inside an email which is sent to people inboxes.
Haven't tried it yet, but perhaps take a look at this, seems like a good practice for embedded images, like a logo.
https://laravel.com/docs/6.0/mail#inline-attachments
Hope it helps!
