It is entirely up to the configuration of the mail server.
Interpretation of the local part of an email address is dependent on the conventions and policies implemented in the mail server.
To indicate the message recipient, an email address also may have an associated display name for the recipient, which is followed by the address specification surrounded by angled brackets, for example: John Smith .
Earlier forms of email addresses on other networks than the Internet included other notations, such as that required by X.400, and the UUCP bang path notation, in which the address was given in the form of a sequence of computers through which the message should be relayed.
The MTA next connects to this server as an SMTP client.
Email aliases, electronic mailing lists, sub-addressing, and catch-all addresses, the latter being mailboxes that receive messages regardless of the local part, are common patterns for achieving a variety of delivery goals.
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.
In some cases they also limit which addresses it is possible to send mail to.
A quoted string may exist as a dot separated entity within the local-part, or it may exist when the outermost quotes are the outermost characters of the local-part (e.g., specify different mailboxes; however, many organizations treat uppercase and lowercase letters as equivalent.
Indeed, RFC 5321 warns that "a host that expects to receive mail SHOULD avoid defining mailboxes where ... Despite the wide range of special characters which are technically valid, organisations, mail services, mail servers and mail clients in practice often do not accept all of them.