RHYMING RIDDLE

I'm found in valley, but not in hill,
In a stream I abide, but not in a rill,
I begin in enemy, and end in foe,
Always welcome wherever I go.

I lie in the heavens, earth, and sea,
In house and home, and always free,
I'm found in sunrise, though I shun its light,
I'm never seen in day or night.

Answer:

Letter E