As @xblitz explains (thanks!), –send-mail-url is what your Internet Service Provider requires for their outgoing mail server. You can ask them or see if they have a web page. Almost all public ISPs require –send-mail-username and –send-mail-password so they know who’s using (or mis-using) their server. –send-mail-to is where you want the email to go to, which could be your email address or some other.
Some other parts (or maybe even the above) get technical, and whoever sends your email can advise. Setting up Duplicati is similar in concept to setting up any ordinary email program that you run locally…
One tricky part is encryption (for your email security), and your server may offer different options which then might show up in what server port you use, and what you say about TLS encryption. Ask your ISP.
smtp://smtpauth.myISPsite.net:587/?starttls=always is what I’m running. Port 587, encrypted.
It can get worse if your ISP has more severe authentication needs than the above username/password.
Viewing the Duplicati Server Logs at About --> Show log --> Live --> Profiling can log the email actions.
2019-12-23 12:08:00 -05 - [Profiling-Duplicati.Library.Modules.Builtin.SendMail-SendMailResult]: Whole SMTP communication: Connected to smtp://smtpauth.myISPsite.net:587/?starttls=always
after the backup itself is finished is the email report trying to get out. Don’t post all the details because it may reveal secrets between Duplicati and email server. Be especially careful near