AFAIK, "To" and "to" headers should be equivalent. The common thing to do, in my experience, is to convert every header to title case, for example "Content-Type" which is the equivalent of any from "content-type", "CONTENT-TYPE", ...

In other words: I think it's a bug in Email::Simple, as it doesn't seem to convert the headers to a standard form. I do see a line

        header_names => { map { lc $_ => $_ } keys %$head_hash }
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in the sub new, but that doesn't change the contents of the hash %$head_hash, which is used verbatim.

        head => $head_hash,
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