Yes. You can use an existing SMTP server (gmail [legacy app mode], yahoo, microsoft, etc). One thing to keep in mind -- if you decide to go that route don't use your personal account because the address might be exposed in the mail headers. Create a dedicated account and use that instead.

lemmy.hjson

email: {
    smtp_server: "smtp.example.tld:[port]" # port 25, port 587, etc
    smtp_login: "username"
    smtp_password: "password"
    smtp_from_address: "noreply@example.tld" # or account_just_created@example.tld
    tls_type: "tls" # or starttls
}

No idea. I've been playing with it all night but so far I haven't managed to get the postfix-relay docker container to connect and send an email.