18.39Mixolydian over a secondary dominant

If E7 is a straightforward dominant resolving to A, one simple scale is:

E Mixolydian

E F♯ G♯ A B C♯ D

But notice:

that introduces F♯ and C♯, which may or may not fit the surrounding C-major melody/context.

So chord-scale theory always needs context.

This is why I keep prioritizing chord tones.