RFC 1428 (rfc1428) - Page 3 of 6
Transition of Internet Mail from Just-Send-8 to 8bit-SMTP/MIME
Alternative Format: Original Text Document
RFC 1428 Transition to 8bit-SMTP/MIME February 1993
characters are in the US-ASCII subset of ISO 8859 may be somewhat
readable.
(3) 8bit transparent sender to 8bit transparent receiver
Will work if an external agreement "out of band" to use a
particular character set without tagging exists between the sender
and the receiver.
(4) 8bit transparent sender to MIME/ESMTP conformant receiver
Will work if a reasonable upgrade path is provided via gateways,
the indicated character set tag inserted by the gateway is correct
and the receiver supports the character set chosen by the sender.
This case is the focus of this memo.
(5) MIME/ESMTP sender to non-MIME 7bit receiver
Because the ESMTP/MIME sender cannot know if the receiver will
understand 8bits, the sender will encode the text into base-64 or
quoted-printable which may be considered "garbled" by the
receiver. To provide a useful downgrade path the gateway must
have some knowledge about the capabilities of the receiver. When
the character set can be clearly identified, techniques like the
menmonic MNEM encoding described in RFC 1345 may be helpful in
this case.
(6) MIME/ESMTP sender to MIME/ESMTP receiver
Interoperability will be attained provided the receiver supports
the character set chosen by the sender.
3. Upgrade Path from 8bit Transparent to ESMTP/MIME
A gateway which has been upgraded to support Extended SMTP may
upgrade an 8bit message received to MIME. This is consistent with
the requirement that all 8bit mail sent by ESMTP be encoded in MIME.
The upgrade should be done using the best available information.
A site may "upgrade" to MIME en-masse by implementing MIME conversion
for all messages leaving the site. For text messages, the body can
be converted by adding a MIME-version header and a Content-Type:
Text/Plain with the character set in use in the site, provided the
site uses a single character set.
An appropriate Content-Transfer-Encoding header line must be added to
indicate any encoding that may be necessary.
Vaudreuil