In order to facilitate password resets, notification e-mails, etc, you will need to provide SMTP access to your Synapse instance. More details - or as reference - see this link.
You will need to edit /opt/matrix/synapse/homeserver.yaml.
public_baseurl: https://matrix.example.com/
smtp_host: example.com (mail.example.com, etc)
smtp_port: 587
smtp_user: "matrix@example.com"
smtp_pass: "SOMETHING COMPLEX"
notif_from: "Your Friendly %(app)s homeserver matrix@example.com"
client_base_url: "http://riot.matrix.example.com"
Restart synapse: docker restart synapse
Restart your element app and test adding an e-mail for your account (Settings -> General -> Emails and phone numbers)