Email addresses aren’t case sensitive so you can use whichever you prefer. John.Doe@gmail.com = email@example.com = jOhN.DoE@gMAiL.com. It’s purely a question of personal preference and aesthetic. Just don’t mix up letters with numbers and the email will still go through.
Hope this helps.
Most SMTP servers are fine with uppercase characters. However, some do differentiate between upper and lower case characters in the recipient part. The domain part is case insensitive.
For example, google mail does not differentiate between upper and lower case recipients.
They are not case sensitive, so it doesn’t matter when sending the email. However, whether or not to capitalize on your website, business cards, etc. is more of a design decision. If there are multiple words in the email address and you want to capitalize for emphasis. Or you might want to use lower-case for a more minimalist look.