Consider this progression:

C - D - E - E

All major triads. So what am I looking at?

VI - VII - I - I ?

I know it's a nondiatonical progression. The Roman Numerals will all be capitalized because the chords are all major triads, right?

VII - I - II - II ?

I thought maybe this because I like using D Mixolydian over it.

D Mixolydian contains the most shared notes with the progression. But beyond this, how can I justify in theory why I'm using it? Is it truly the key center?
Your key is E major. Your just using chords from the parallel minor (e minor)

Quote by Gunpowder
Thrashturbating? Most metal of all ways to pleasure oneself.
Quote by griffRG7321
bVI bVII I in E major.

Ah, I see now. The chord names are major, but their relation to the root chord is flat.