Understanding the Connection Log / SMTP protocol

In order to understand MaxBulk Mailer "Connection logs" you need to read RFC 821 (Request for Comments 821). It is a technical memo about SMTP protocol implementation. (SMTP stands for SIMPLE MAIL TRANSFER PROTOCOL).

SMTP protocol is a text protocol where a SMTP client (MaxBulk Mailer) talks to a SMTP server. Basically this conversation looks like this (This example shows how the client connects, send the sender data, then the recipient data and finally the message itself):

Server:		220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
Client:		HELO MIT-MC.ARPA
Server:		250 SU-SCORE.ARPA
Client:		SEND FROM:<[email protected]>
Server:		250 OK
Client:		RCPT TO:<[email protected]>
Server:		450 User not active now
Client:		DATA
Server:		354 Start mail input; end with <CRLF>.<CRLF>
Client:		Blah blah blah...
Client:		...etc. etc. etc.
Client:		.
Server:		250 OK
Client:		QUIT
Server:		221 SU-SCORE.ARPA Service closing transmission channel

MaxBulk sends commands like:

HELO <domain>
RCPT TO: <forward-path>
DATA
SEND FROM: <reverse-path>
QUIT

The server always answer with a 3-digits code and a message. For example:

211		System status, or system help reply
220		<domain> Service ready
221		<domain> Service closing transmission channel
250		Requested mail action okay, completed
251		User not local; will forward to <forward-path>

and if an error occurs, the server message always starts with a 4xx or 5xx code like:

421		<domain> Service not available, closing transmission channel
450		Requested mail action not taken: mailbox unavailable
451		Requested action aborted: local error in processing
452		Requested action not taken: insufficient system storage
500		Syntax error, command unrecognized
501		Syntax error in parameters or arguments
502		Command not implemented
503		Bad sequence of commands
504		Command parameter not implemented
550		Requested action not taken: mailbox unavailable
551		User not local; please try <forward-path>
552		Requested mail action aborted: exceeded storage allocation
553		Requested action not taken: mailbox name not allowed
554		Transaction failed

For more information take a look at the SMTP response code list.