The size of a message is considered to be the number of bytes in its headers and body. That size can be declared or calculated. It is predeclared with the SIZE= MAIL FROM: ESMTP parameter:

MAIL From:<liu@td.co.jp> SIZE=45621

Immediately after sendmail reads the size value from the SIZE= parameter, it stores that value in the ${msg_size} macro. The value is stored before checks for validity are made and so can contain nonnumeric characters. If the message lacks a SIZE= parameter, the ${msg_size} macro will be undefined (NULL).

The message size is calculated again after the entire message has been read (either from standard input, the queue, or via SMTP), and the value in ${msg_size} is updated with that new value. If an external MILTER program (Section 7.6) is called, the ${msg_size} is updated again because that program might have changed the size of the message.

The ${msg_size} macro can be useful in the check_data rule set (check_data), which is called just after the SMTP DATA command and can be used to check the size specified with SIZE=. It can also be useful in the check_compat rule set (Section 7.1.4), which is called just before delivery and can be used to check the size of the received message.

${msg_size} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{msg_size}, not ${msg_size}).