is correct to use "Many thanks"?
What is the context for use?

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Answer

You can say "Thank you very much!"; "Thank you so much!" or simply "Many thanks!" when you want to thank someone.
All examples are right. 
So, "many thanks" is perfectly proper, grammatically, standard English.

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I often use it to end a semi-formal email in a friendly tone.

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I think it is better to say "Thanks so much" or "Thank you very much" )) It is grammatically correct.