Nobody expects emails to be treated case-sensitively. Which is why I found it so surprising that Mozilla would treat it so strictly by the book (titled RFC822 actually).
At the current time of writing, Thunderbird stands at v126.96.36.199 and after 7 or more years of a bug track being posted, it still isn't fixed. =\
If you've got a Mozilla bug tracker account, please vote it up so this obsolete requirement is removed!
Direct link to vote is here.