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RFC 1870 / SMTP MESSAGE SIZE

Discover the server’s size policy

01

SMTP is the protocol mail systems use to transfer messages. EHLO asks which extensions the server supports; SIZE can also advertise a fixed byte limit.

DIAGRAMRead RFC section 3 ↗
RFC 1870 · § 3EXAMPLE
4 / 4
Discover the server’s size policyFour separate EHLO outcomes: SIZE 1000000 advertises a chosen fixed limit, SIZE 0 means no fixed maximum but finite resources, bare SIZE omits fixed-limit information and requires resource checking before positive declared-size acceptance, and no SIZE advertisement means this extension is unavailable. Successful EHLO replies in these examples are schematic; other capabilities are omitted.Ask what this server supportsSMTP clientSMTP serverEHLO client.example250 HELPNo SIZE in the EHLO replysend MAIL with SIZEUse ordinary SMTP without SIZEOrdinary transfer failures remain possible
04
Do not assume support.

A successful EHLO reply without SIZE does not advertise this extension. Use ordinary SMTP without a SIZE parameter; ordinary errors remain possible.

Source for this drawing: RFC 1870 · Read RFC section 3 ↗

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A message needs an address.

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

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KEY POINT

Support, a fixed limit, and available storage are different facts.

  • SIZE with a positive decimal value advertises a fixed maximum; SIZE 0 means no fixed maximum is in force.
  • Bare SIZE advertises support but conveys no fixed-maximum information. Without SIZE support, do not use its extended MAIL parameter.
  • For bare SIZE, a server may not positively accept a declared size before checking transfer and stable-storage resources; if possible it should reserve storage.
DETAILS

Zero does not promise unlimited storage. A minimal finite-limit server need not detect temporary limits early.

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