What is this tree?

It looks like Russian Olive to me.

The Olive tree is recognized because the leaves have a silvery underside and teeth-like edges. It can live thousands of years, seriously. A year ago I visited the Gardens of Gethsemane at the foot of the Mount of Olives in Jerusalem and some of the trees have been dated back to the times when Jesus and His disciples walked around those grounds. So, it is likely you have gone by olive trees that have seen the earlier pilgrims!

Southern Spain has acres and acres and acres of olive farmlands. I love them!