client
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... 5] to define extensions to
the SMTP service whereby a client ("sender-SMTP") may declare the
...
... receiver-SMTP"), after
which the server may indicate to the client that it is or is not
willing to accept the message based on the declared message size and
...
... receiver-SMTP") may declare the maximum message
size it is willing to accept to a client ("sender-SMTP").
...
... The remainder of this memo specifies how support for the extension
affects the behavior of an SMTP client and server.
...
... SMTP server may have a fixed upper limit on message size. Any
attempt by a client to transfer a message which is larger than this
fixed upper limit will fail. In addition, a server normally has
limited space with which to store incoming messages. Transfer of a
...
... might succeed at a later time.
A client using the unextended SMTP protocol defined in [1], can only
...
... transmitting the entire message to
the server (which discards the transferred message). If, however,
both client and server support the Message Size Declaration service
...
... EHLO
response, it indicates the size of the largest message that the
server is willing to accept. Any attempt by a client to transfer a
message which is larger than this limit will be rejected with a
permanent failure (552) reply code ...
... MAIL command described below.
When supported by the server, a client may use the extended MAIL
command (instead of the MAIL command as defined in [1 ...
... SMTP DATA command's terminating dot or doubled
quoting dots, to be transmitted by the SMTP client after receiving
reply code ...
...
The declared message size is defined as a client's estimate of the
message size for a particular message.
...
...
The extended MAIL command is issued by a client when it wishes to
inform a server of the size of the message to be sent. The extended
MAIL command is identical to the MAIL command ...
... However, since exact computation of the message size may be
infeasable, the client may use a heuristically-derived estimate.
Such heuristics should be chosen so that the declared message size ...
... extended MAIL command, such
as might occur if the client employed a size-estimation heuristic
which was inaccurate.
...
...
The client, upon receiving the server's response to the extended MAIL
command, acts as follows:
...
...
(1) If the code "452 insufficient system storage" is returned, the
client should next send either a RSET command (if it wishes to
attempt to send other messages) or a QUIT command. The client ...
... client should next send either a RSET command (if it wishes to
attempt to send other messages) or a QUIT command. The client
should then repeat the attempt to send the message to the server
at a later time.
...
... (2) If the code "552 message exceeds fixed maximum message size" is
received, the client should immediately send either a RSET
command (if it wishes to attempt to send additional messages),
...
... additional messages),
or a QUIT command. The client should then declare the message
undeliverable and return appropriate notification to the sender ...
... extended MAIL
command does not constitute an absolute guarantee that the message
transfer will succeed. SMTP clients using the extended MAIL command
must still be prepared to handle both temporary and permanent error
reply codes ...
... message size.
A client is permitted to declare a message to be smaller than its
actual size. However, in this case, a successful (250) reply code is
...
... accept a message of the declared size for a particular recipient.
(1) If a 452 code is returned, the client may requeue the message for
later delivery to the same recipient.
...
... delivery to the same recipient.
(2) If a 552 code is returned, the client may not requeue the message
for later delivery to the same recipient.
...
...
A "minimal" client may use this extension to simply compare its
(perhaps estimated) size of the message that it wishes to relay, with
the server's fixed maximum message size ...
... A minimal server that employs this service extension may simply use
the SIZE keyword value to inform the client of the size of the
largest message it will accept, or to inform the client that there is
...
... the SIZE keyword value to inform the client of the size of the
largest message it will accept, or to inform the client that there is
no fixed limit on message size. Such a server must accept the
...
... extended MAIL command and return a 552 reply code if the client's
declared size exceeds its fixed size limit (if any), but it need not
detect "temporary" limitations on message size ...