The domain name part such as
[email protected] is not case sensitive so it doesn't matter if you use uppercase or lower case letters in the email address.
The domain part of the email address would be case insensitive but for the password to the yahoo email account that would be case sensitive and you need to make sure that you enter the password to the yahoo email account correctly.
If you can't remember the yahoo email account password then you can reset the password by verifying some information.