Yahoo email addresses and other email addresses are not case sensitive.
So you can type in the username of the email address and the domain of the email account in either lower case letters or upper case letters and it won't matter.
I've always typed in email address as lower case letters and never had any issue sending the email.
This is for yahoo email accounts as well as Google email accounts and any other email accounts.
It does not matter at all as to whether you type in the email address with uppercase letters or lowercase letters the email will go through just fine.