The local-part of an addr-spec in a mailbox specification (i.e., the host's name for the mailbox) is understood to be whatever the receiving mail protocol server allows. For example, some systems do not understand mailbox references of the form "P. D. Q. Bach", but others do.
This specification treats periods (".") as lexical separators. Hence, their presence in local-parts which are not quoted-strings, is detected. However, such occurrences carry NO semantics. That is, if a local-part has periods within it, an address parser will divide the local-part into several tokens, but the sequence of tokens will be treated as one uninterpreted unit. The sequence will be re-assembled, when the address is passed outside of the system such as to a mail protocol service.
For example, the address:
is legal and does not require the local-part to be surrounded with quotation-marks. (However, "First Last" DOES require quoting.) The local-part of the address, when passed outside of the mail system, within the Registry.Org domain, is "First.Last", again without quotation marks.