What does it mean for a guy ( that I went to middle school with) sent me a letter through mail?

I found it weird, This guy was an acquaintance and we barely knew each other after middle school. He found my home address and sent me a formal letter. Formal, as in, something you would write at work to your CEO. I'm 23 rn. I have social media that he could have contacted me through. But no mutual friends.

Just wondering what others would think in this situation and if this is a normal thing.

What does it mean for a guy ( that I went to middle school with) sent me a letter through mail?
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