I was able to get Brevo working on Hetzner with the following block:
email: {
smtp_server: "smtp-relay.sendinblue.com:587"
smtp_login: "your_email"
smtp_password: "smtp_key"
smtp_from_address: "your_email"
tls_type: "starttls"
}
Interestingly, it wasn't working when I gave up and went to bed. I happened to do something on the instance today and much to my surprise, an email came through. Maybe Brevo holds the emails/doesn't immediately allow SMTP?
if that hadn't worked, my next step was going to test sending email through the console as described in another comment.
I tried Navidrome but am now very happy with Plex and Plexamp.