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Glossary · SMTP

8BITMIME

An SMTP extension (RFC 6152) that lets a client send message bodies containing 8-bit bytes, such as raw UTF-8 text, without encoding them first.

What it means

Plain SMTP carries 7-bit ASCII. A server that announces 8BITMIME accepts BODY=8BITMIME on MAIL FROM, meaning the body may contain octets above 127 in lines of ordinary length. The header still has to be ASCII unless SMTPUTF8 is also in use.

When the next server does not support 8BITMIME, the sending server must either convert the content to a 7-bit encoding, quoted-printable or base64, or return the message as undeliverable. Conversion changes the bytes of the body and so breaks any DKIM signature made over the original, which is why RFC 6376 advises signers to encode 8-bit content as quoted-printable or base64 before signing.

Sources

What this definition rests on. The wording is ours; the facts are theirs.

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