@Groo
If the emails are going into your Spam folder it means that the Spam filters are working on them but have been unable to say 100% if it is spam or a genuine email and as such place it into your Spam folder to let you decide.
You do not need to open an email to block the email address or domain, although if it is spam it is virtually a waste of time doing that because the email address will probably be spoofed and the spammer uses hundreds if not hundreds of thousands of spoofed email addresses daily. They also set up and use "disposable" domains as well as genuine domains.
Be-aware that a spoofed email address can be sent from a genuine domain such as Google and if you block that domain all emails from the Google domain will be blocked.
If you do want to block the email address without opening it simply select the errant email by placing a tick in the box on the left side of the email in the list. To do this DO NOT double click, just single click.
Once selected go to the "More" button in the menu bar and then drop down to "Block Sender" or Block Domain" or "Mark as Spam" and select one of the three options offered.
Unfortunately this method is greyed out if multiple options emails are selected at the same time.
You can check the emails source by selecting it then going to the "More" button and selecting the "View Source" option. This will let you see where the email originated from and its genuine email address and domain.
Email spoofing - Wikipedia
Email spoofing: how attackers impersonate legitimate senders | Securelist
