Message Size Declaration

An SMTP server may have a maximum size of message it can receive. In the un-extended SMTP, a client is informed that a message he tried to send exceeded the maximum size of a message only after the whole message was sent. The SIZE extension uses the ESMTP extension format to overcome this issue.

How it works
Like in every usage of an ESMTP extension, the SMTP session should be initiated with the EHLO command. In case the server supports the SIZE extension, it will be listened in the response of the server. Optionally, an integer value can be appended to the SIZE command in the list, specifying the maximum message size supported by the server.

Next, the client should use the Extended MAIL command that adds a size field to the original MAIL command. If the value that was given by the client is bigger than the maximum size supported by the server, then his command is rejected with a permanent failure (552) reply code.