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RFC 1870 / SMTP MESSAGE SIZE

Count message octets, not transport dots

02

An octet is one eight-bit byte. SIZE counts the transmitted message headers, blank line, body and CRLF line endings, before SMTP dot quoting.

DIAGRAMRead RFC section 5 ↗
RFC 1870 · § 5EXAMPLE
4 / 4
Count message octets, not transport dotsA tiny ASCII counting fragment has Subject colon space Hi CRLF, an empty CRLF line and dot Hello CRLF:23 octets. It is not a complete RFC5322 message. Dot transparency adds one dot and the final dot CRLF adds three octets:27 wire octets after354, while SIZE remains23. MAIL FROM places SIZE after the envelope address. The decimal grammar permits1–20 digits, shown with an exact20-digit integer, not a guaranteed acceptable size.Count message bytes before dot quotingSIZE=99999999999999999999Value grammar: 1–20 decimal digitsMAIL line allowance: +26 charactersNo new SMTP verbExact decimal · do not round itValid syntax does not imply acceptance
04
Keep long decimal values exact.

The grammar allows up to 20 decimal digits. The chosen 99999999999999999999 example must not be rounded through floating-point arithmetic; it is syntax, not a promised acceptable size.

Source for this drawing: RFC 1870 · Read RFC section 5 ↗

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A message needs an address.

Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.

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RFC 5321 SMTP RFC 5322 Email message format RFC 2045 MIME message bodies

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Related RFCs and changes

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KEY POINT

A message byte count is not a file size or a count of displayed characters.

  • Include headers, body and each two-octet CRLF; exclude the terminating dot and added transparency dots.
  • MAIL uses one SIZE parameter with 1–20 decimal digits. Its command-line allowance grows by 26 characters; no new verb is added.
  • The declared size may be a heuristic estimate. Ideally exact, estimates should usually exceed the actual message size.
DETAILS

This tiny ASCII example is a counting fragment, not a complete RFC 5322 message. MIME-encoded body bytes, not an attachment’s original size, count.

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