In short, Email addresses ARE NOT case sensitive and Passwords ARE case sensitive.
Email Addresses:
When creating usernames for your email accounts, the username should be created in all lowercase letters. When the username is created in this manner, the recipient will receive the email despite the capitalization of the username. However, even if you do create it with UPPERCASE letters it will not matter anyway. It is just good practice to keep it lowercase. Passwords are case sensative. Make sure you clearly document your passwords as this will affect the users ability to login. If not entered exactly as it was created, access will be denied.
For example, if the username is created as mymail@domain.com, then the recipient can receive email if the email address is entered as MyMail@domain.com, Mymail@domain.com, mymail@domain.com or any other deviation of the letter format.
Password:
Passwords are case sensitive. If you create a password with the password of “REST”, then the entering the password of “rest” in your email client/program, will result in an error.