of an incoming SMTP message is the number of bytes sent during the
DATA phase (See this section), including end-of-line
characters. The size of a message received via
sendmail's standard input is a
count of the bytes received, including the newline characters. In
both instances the size is displayed with the
size= syslog equate:
Note that this size is reported before sendmail
adds or deletes any headers. Therefore, for mail being relayed
through a site, the size will usually be small coming in and somewhat
larger going out.
The size= syslog equate is
produced only for sender records.