Is it true that "thanks a million" is usually used with a slightly negative connotation? I often use it to end friendly emails. But I have seen somewhere that it has a negative connotation as in this example "You've finished my chocolates, thanks a million!" I hope people didn't get offended when I used it in emails, my intention was positive...
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Welcome to English Language & Usage. You might wish to distinguish between written and spoken when asking about this phrase. In the example you give, "thanks a million" might be a positive statement said to one who removed the temptation of chocolates. There is no telling without context. – J. Taylor Mar 1 '18 at 11:25
Thanks a million is just an idiomatic expression meaning "thank you very much".
It can be used sarcastically as any other form of saying "thank you", but I don't think it has a special negative connotation in itself.