It’s a pretty simple “acknowledged” to me. It’s a “I’ve not just seen your message, I’ve read it, and I have no further comments”.

I don’t think I’ve ever interpreted it as rude.

Seinfeld for me.

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I didn’t, for me it was “Ai, Bee, See, Dee, Eee, Eff, Jee” (except in my local language Danish). My children all learnt phonics in their U.K. school and it’s taught them to read 5x faster I’d say).