Login to your email account and go to the settings.
For Gmail:
You will find a section for blocked or filtered email addresses.
You want to add a new filter which will allow emails from @physics.ox.ac.uk to always be seen in your inbox.
Add the string of text @physics.ox.ac.uk into the box for the email address to be filtered, then apply the rule "never goes to spam" or similar.
Create the filter.
For Outlook:
Find Mail in Settings and look for Junk Mail.
Click on +Add under Safe senders and domains to add @physics.ox.ac.uk to the allowed list.
Close the Settings window
For Yahoo
Navigate to Settings icon, then click More Settings from the Yahoo Mail menu.
Select Filters and hit Add new filters to enter information about the domain name or sender.
enter the string of text you want to whitelist in future emails, @physics.ox.ac.uk
Save the filter.