D. J. Bernstein
Internet mail
SMTP: Simple Mail Transfer Protocol

The SIZE extension

SIZE was introduced in RFC 1427, published in 1993. RFC 1427 was replaced by RFC 1653, published in 1994, and then RFC 1870, published in 1995.

The SIZE extension has two purposes:

To give the server an estimate of the size of a message before the message is transmitted.
To warn the client that messages above a certain size will not be accepted.

The claimed benefit is an occasional large reduction in SMTP traffic. Beware, however, that using SIZE also means a persistent small expansion in SMTP traffic. Measurements at several hosts have found that the expansion outweighs the reduction.

The SIZE extension has at most one argument. If the argument is supplied, it consists entirely of ASCII digits, giving a number n. If n is nonzero, the server is indicating that it will reject messages containing more than n bytes (counting two bytes per line for \015\012).

Whether or not an argument is supplied, the SIZE extension has the following meaning. During this connection, the client may send the following string, between 7 and 26 characters, as extra information at the end of a MAIL request:

a space;
the word SIZE;
an equals sign;
between 1 and 20 ASCII digits, giving a number.

The number is an estimate, usually but not necessarily an overestimate, of the total number of bytes in the message.