Usernames often contain both upper-case and lower-case letters and may also contain numbers, underscores, or periods.
Does Case Matter?
Mostly No
Is recipient@example.com the same as Recipient@EXAMPLE.com as far as email servers are concerned?

Lifewire / Theresa Chiechi
Lifewire / Theresa Chiechi
Google email addresses ignore the letter case and periods.
For example, jsmithe@gmail.com is the same as j.smithe@gmail.com.
Not many email services or ISPs enforce case-sensitive email addresses.