SMTP Service Extension for 8bit-MIMEtransport
obsoleted by
- rfc-6152 — SMTP Service Extension for 8-bit MIME Transport
obsoletes
- rfc-1426 — SMTP Service Extension for 8bit-MIMEtransport
Extracted elements (13)
Capability negotiation uses the existing EHLO mechanism from RFC 1651 rather than a new command, keeping the extension framework consistent with other SMTP extensions.
The extension does not eliminate line length limits, so it does not provide a general mechanism for transferring unencoded binary data via SMTP. This is a deliberate scope limitation: the BINARY content-transfer-encoding remains out of scope.
When a server does not support 8BITMIME, a client has two options: implement a gateway transformation to convert the message to valid 7bit MIME, or treat it as a permanent error and handle it as a delivery failure.
A server that supports the 8-bit MIME transport service extension shall preserve all bits in each octet passed using the DATA command.
Conversion from 8bit MIME to 7bit MIME must cause no loss of information — MIME transport encodings must be employed as needed — and the resulting message must be valid 7bit MIME.
If a server SMTP does not support the 8-bit MIME transport extension, the client SMTP must not, under any circumstances, attempt to transfer content containing characters outside the US-ASCII octet range (hex 00-7F).
Line length restrictions still apply even when using 8BITMIME: servers may limit lines to as few as 1000 octets between two CR-LF pairs. This extension MAY support 8BIT content-transfer-encoding but does NOT provide a means of transferring BINARY content-transfer-encoding.
Once a server SMTP supporting 8bit-MIMEtransport accepts a content body with the high-order (8th) bit set, the server SMTP must deliver or relay the content in a way that preserves all bits in each octet.
The standard SMTP data-stuffing algorithm must be applied: a content containing <CR><LF><DOT><CR><LF> or beginning with <DOT><CR><LF> must be escaped to avoid prematurely terminating the transfer.
An optional BODY parameter is added to the MAIL FROM command. Its value is either '7BIT' (strict RFC 821 compliance) or '8BITMIME' (MIME with arbitrary octet content). Only one BODY parameter may appear in a single MAIL command.
The SMTP service extension is named '8bit-MIMEtransport'; the EHLO keyword value is '8BITMIME'. No parameter is used with the 8BITMIME EHLO keyword. A server advertising this keyword indicates it will accept MIME messages containing arbitrary octet-aligned material.
This RFC does not introduce new security issues beyond those already endemic in electronic mail and present in fully conforming implementations of RFC 821 (SMTP).
The BODY parameter value syntax is: body-value ::= "7BIT" / "8BITMIME". The esmtp-keyword is BODY and the extended MAIL FROM syntax follows the framework defined in RFC 1651.