These days the world assumes that all parts of emails are case-insensitive, even if RFC5321 says otherwise. If it’s true for Google, Outlook & Apple mail then it’s basically true everywhere & everyone else has to get with the program.

If you don’t want to lose potentially important email then you need to make sure your own systems are case-insensitive everywhere. Otherwise you’ll find out the hard way when a customer or supplier is using a system that capitalises entire email addresses (yes, I have seen this happen) & you lose important messages.