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For some hosts the user name is case sensitive, and SMTP implementations must take case to preserve the case of user names as they appear in mailbox arguments. Host names are not case sensitive.

--RFC821 : Simple Mail Transfer Protocol

Some email addresses are case insensitive--probably most are these days--but there is no guarantee that every email address you encounter "must" be case insensitive.

Part of the problem comes from confusion with URLs, which to many people are "internet thingies" just like email addresses. URLs are partly case sensitive, but only sometimes. The protocol//host:port is not, but the directories can be depending on which OS the server is running. This is quite a mess for people to keep track of, so the safe assumption is that *everyting* is case sensetive, since to not assume so will probably erase their computers somehow.

The other problem, it that people often InterCap their URLs and email usernames to make them more legible, and, as a newbie the user learned that any deviation from the letter of the instructions results in disaster.

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