1. "Stop doing that" is a standard phrase. It means "Stop doing the thing that you are doing". If someone said to me "Stop doing this", I would expect them to demonstrate the action they want me to stop.
2. "Mail" is a noun but it is generally accepted, in this phrase, to mean "an email" or "some emails". It is a collective noun. It does not need to take the article. "You got" is the equivalent simple past of "You received". Grammatically, it should be "You have got mail".
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