just a short information: All entries in mp-content/advanced/html2txt/entities.xml contain the regex option "i" for case insensitive pattern matching.
This is erroneous. Just a little example for German umlauts, which exist in upper and lower case:
Language file contains: "
Result in mail subject: "TÄglicher Newsletter" instead of "Täglicher Newsletter"
Cause: The first pattern in entities.xml that matches is "
Ä" due to the caseless option.
In my opinion no entity should be matched case insensitive. If someone writes the HTML entity in wrong case, it's definitly his own fault.