I have an openSUSE email alias. When I write down the email address, most of the time, I put the "suse" part capitalized to preserve the correct lettering of openSUSE. Domain names are case insensitive. Therefore if you type opensuse.org or openSUSE.org in your browser, they would both be accepted. When people reply my emails they would notice the address bar writes @openSUSE.org which contains the capitalized part.
When I receive the email though, it is turned to @opensuse.org. I was thus murmuring 'damn, case insensitive' when I was reading my emails a few days ago at meeting. The gentleman next to me murmured back, 'no, emails are case sensitive'. I argued no, even if you write the address capitalized the email server interprets and replies back with a full non-capitalized address.
The gentleman was S. Moonesamy. He slapped RFC 5321 at my face & that was like 'do your homework'.
ish != Ish
Section 2.4 of RFC 5321 speaks about syntax principles. It mentions the local-part of a mailbox must be treated as case-sensitive since that would be the case for usernames. The user "ish" would be different from user "Ish". It is advised that SMTP implementations take care of it to avoid interoperability issues. The same paragraph also mentions that mailbox domains follow normal DNS rules and are not case sensitive.
Domain names are case insensitive and are rendered non-capitalized. Usernames are case sensitive but email servers treat them as case-insensitive and take care of the local naming to ensure delivery. Thus emails to firstname.lastname@example.org or UserName@DOMAIN.com will get delivered to the same mailbox.