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There's no way to know the "correct" case of an email local-part. That's the definition of case-sensitivity. All you can do is ensure that all of your imports and form posts are case-preserving (don't change what is originally provided, assume it's the most deliverable form).
The number of addresses that are case-sensitive and contain upper-case letters is small -- that is, even when a mailserver is case-sensitive modern-thinking admins would try to give users an all-lowercase form -- but it's not zero.