However, you cannot restrict the usage of particular email sources.
Messages you receive should be interpreted as the sender intended so long as the meaning is clear.
This article uses the term email address to refer to the addr-spec defined in RFC 5322, not to the address that is commonly used; the difference is that an address may contain a display name, a comment, or both. [email protected] made up of a local-part, an @ symbol, then a case-insensitive domain.
Although the standard specifies the local part to be case-sensitive, in practice the mail system at may treat John.
Smith as equivalent to John Smith or even as johnsmith, and mail systems often limit their users' choice of name to a subset of the technically valid characters.