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Meaning 1

I’ll tell you later

Context 1

“Is the meeting still on for tomorrow?”
“No I cancelled it yesterday, I sent an email round”
“Oh so when will it be on?”
“I’ll let you know”
“ok”

Meaning 2

To tell you (politely)

Context 2

(email)
“Just to let you know that reception will close early today, 4.30, thanks”