An email address identifies an email box to which email messages are delivered. The universal standard for the format and meaning of an email address today is the model developed for Internet electronic mail systems since the 1980s, but some earlier systems, and many proprietary commercial email systems used different address formats. An email address such as John.Smith@example.com is made up of a local part, an @ symbol, then a domain part. The domain part is case-insensitive but the local part is (in theory) case-sensitive. In practice, the mail system at example.com may (and most mail systems will) treat John.Smith as equivalent john.smith or even johnsmith. Mail systems often limit their users’ choice of name to a subset of the technically valid characters, and may in some cases also limit which addresses it is possible to send mail to. With the introduction of internationalized domain names, efforts are progressing to permit non-ASCII characters in email addresses.