Hello Worktomuch,
While you can't allow a single email address to email you with a filter, you can allow a specific email account using these steps:
Add Focus filters
When you set up a Focus, you can add app filters that determine what information apps will show when the Focus is on. For example, you can show your work calendar only while your Work Focus is on.
1.) Go to Settings > Focus, then tap the Focus you want to add filters to.
2.) Scroll down to Focus Filters, then tap Add Filter.
3.) Tap an app, then select the information you want to see from that app while the Focus on.
4.) Tap Add.
You can read more here: Use Focus on your iPhone or iPad - Apple Support
If you'd like to leave feedback regarding this feature, you can do so at the following link: Product Feedback - Apple
Take care.