An SMTP relay would usually defer verification of addresses when service extensions are specified that require verification with the destination host. To be able to provide the relay capability the server SMTP is supplied with the name of the ultimate destination host as well as the destination mailbox name. The argument to the RCPT command is normally also an address, which specifies who the mail is to. When the same message is sent to multiple recipients the SMTP encourages the transmission of only one copy of the data for all the recipients at the same destination host. The mail commands and replies have a rigid syntax.
Replies also have a numeric code.
In the following, examples appear which use actual commands and replies. The complete lists of commands and replies appears in Section 4 on specifications. Commands and replies are not case sensitive. That is, a command or reply word MAY be upper case, lower case, or any mixture of upper and lower case. Note that this is not true of mailbox user names.