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A server capable of replying with an Enhanced Status Code MUST preface (prepend) the Text Part of SMTP Server responses with the Enhanced Status Code followed by one or more spaces. For example, the "221 Bye" reply (after QUIT command) MUST be sent as "221 2.0.0 Bye" instead. [1]
Callback verification can still work if rejecting all bounces happens at the DATA stage instead of the earlier MAIL FROM stage, while rejecting invalid e-mail addresses remains at the RCPT TO stage instead of also being moved to the DATA stage. [2] [3] Servers that accept all e-mail address at RCPT TO stage but reject invalid ones at DATA stage.
An SMTP transaction consists of three command/reply sequences: MAIL command, to establish the return address, also called return-path, [24] reverse-path, [25] bounce address, mfrom, or envelope sender. RCPT command, to establish a recipient of the message. This command can be issued multiple times, one for each recipient.
LMTP must not be used on port 25, reserved for SMTP; LMTP uses LHLO verb instead of EHLO or HELO used in SMTP; ESMTP requires a single status for the entire message body. LMTP requires a response for each previously successful RCPT command.
In fax and voice data modes, sometimes events occur that the modem wishes to signal to the computer regardless of whether it's in command or data mode. An example of such an event is a caller pressing a touch-tone key in voice mode. Other common out-of-band messages are notifications from the modem that data was lost because the computer is ...
Note that most command-line FTP clients present their own non-standard set of commands to users. For example, GET is the common user command to download a file instead of the raw command RETR . Command
Instruction class - indicates the type of command, e.g., interindustry or proprietary INS 1 Instruction code - indicates the specific command, e.g., "select", "write data" P1-P2 2 Instruction parameters for the command, e.g., offset into file at which to write the data L c: 0, 1 or 3 Encodes the number (N c) of bytes of command data to follow
The method for recording audio data is the same, except that the command is AT+VRX, or AT#VRX, and the modem transmits audio data while the computer receives it. The RTS/CTS flow control are not used here (the computer must accept all the audio data it receives, and the modem automatically paces its transmission to match the audio sampling rate).