When writing an email we probably all check the content of the email for spelling and grammar mistakes but rarely do we double-check the email address.
Email addresses can be broken down into three distinct parts, the part before the @ symbol(the email username), the @ symbol and the part after the @ symbol (the email host), like so firstname.lastname@example.org.
And according to RFC1035, the email host part of an email address is never case sensitive.
However, e-mail addresses are not usually case-sensitive because of the difficulties and confusion it would cause between users, the server, and the administrator. Generally speaking, we can assume that all email addresses are not case sensitive. The bigger Email providers such as Gmail, Hotmail/Outlook, Yahoo Mail all act under this assumption. That being said it is always best to use an email address with the correct casing just to be safe and to ensure that the correct recipient receives the email.
I have tried various versions of my own email address such as these:- 'wILL@fATBUDDHAdESIGNS.CO.UK' and 'Will@FatBuddhaDesigns.co.uk', I can verify that both these both work with my hosting company.
A quick note about Gmail email addresses apart from case sensitivity, is that they are not sensitive to dots/periods. Consider the email address email@example.com, if you were to send an email to firstname.lastname@example.org this would still get through.
In conclusion, I would advise when creating a new email address to follow convention and use all lower-case characters, but in practice, this does not matter.