How can I change email envelope from email from www-data@hostname to user1@hostname?

To prevent your emails from going straight to the spam/junk folder, I would recommend configuring an SPF record: How To use an SPF Record to Prevent Spoofing & Improve E-mail Reliability | DigitalOcean. Also rename your droplet using the control panel to a valid FQDN such as mail.domain.com depending on what your MX record is set to.

As for changing www-data@hostname to username@hostname, you will need to set a custom From header. If you’re using php’s mail() function, your code should look like this:

mail($to, $subject, $message, "From: username@hostname\r\n");