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Here's my extreme not-a-native-English-speaker temporary advice: for the time being try to avoid "can" and "may" altogether and use "could" instead. You rarely could go wrong with "could". And try to...

The word "may" is slowly dying in English and is being replaced by "can" in most uses.

In your example you could also say "How could you say that? (with heavy stress on the word "say") to express...