We already had the issue that our users open an email the second time and then the mail format is lost with no HTML code in the mail anymore just plain text.
There is a Microsoft specificiation called the "Best Body Retrieval Algorithm", and Util.cpp seems to implement this. Specification version 2018-10-01, section 184.108.40.206 step 3 shows a 10-choice decision table (spans page 9 & 10). According to this table, the case of
shall lead to plaintext being declared the "best" body type (row 10).