An old mailbox is slightly leaning. It looks like the Leaning Tower of Pisa. I wonder why it is leaning so much. sembra naturale?

Hello!
on the “slightly leaning” part of your sentence it’s ok to use “slightly leaning” but you will hear “slighted titled” by a lot more native English speakers, or at least i’ve heard it more, I would recommend using “slightly tilted” because “slightly leaning” sounds a bit awkward to me.
Also I would recommend using visualization so you can give whoever your talking to a visualization, a mental image, about what you are talking about. The sentence I would use is “ An old red mailbox is slightly tilted toward the left.” I use toward because it’s talking about a direction which in this case it’s tilted to the left.
It all depends what you want to use but that’s what recommend.

Extra note: In the United States people use “toward” while other countries like the United Kingdom use “towards” both mean the exact same thing it’s just the “s” that makes a difference.

Good job on the “It looks like the Leaning Tower of Pisa.” part that is a good visualization.

Also watch out for punctuation. you use “?” when you are asking a question like “who?” “what?” “when?” “where?” “why?” in your sentence you asked “…why…” so a question mark should be added at the end.

Hope this helps! :)