Joined the author on their server and when a mail is received by a player who then rejoins if the senders name had a capital letter in it it would change to a lower case in the "from" field and using the reply button would fill in the all lowercase player name.
One other thing to note is the changing of name case does not change the clients received mail. In the video i started with all lowercase name then rejoined with an uppercase and still could view the lower case named accounts mail. This might cause issues with those who share an account and play under different usernames such as siblings.
Email test video:
https://youtu.be/OYj-_OFZYAc