I have the same problem on my Magento 2 install. Transactional emails show up as coming from some garbledy gook email that is associated with the actual web server's user. It's ugly and doesn't scream professional at all. Other emails, such as a contact-us form, don't have the same problem. I've heard you can fix this using an SMTP extension, but I've had problems with emails not working at all when I use those. If that's the only solution I'll see if I can get it to work. But why is this the default functionality for an eCommerce platform?
There are a few solutions that are possible here.
From your admin dashboard, navigate to Stores > Configuration > General > Store Email Addresses. From there, make sure your email addresses are all set properly.
There are also a few sender settings in Configuration > Sales > Sales Emails.
If all of that is set, check your php.ini for Sendmail settings. There's also /etc/hosts/ and /etc/mail/, but that may be dependent on your system.
When you get emails that look like they're coming from a system user (on Linux at least, IDK about Magento on Windows) it's a safe bet that the site is using a local mail relay like sendmail or Postfix. The easiest fix is to have Magento use a 3rd party API for mail sending, at this point there are a lot of them out there that have free usage tiers, and many that have Magento plugins already built.