Content-Transfer-Encoding
The MIME header that states how a part's bytes were encoded for transport: 7bit, 8bit, binary, quoted-printable or base64.
What it means
Email was built for 7-bit text in short lines, so content that does not fit is encoded. RFC 2045 defines the values: 7bit, 8bit and binary declare that the data is not encoded and say what kind it is; quoted-printable and base64 are real encodings. When the header is absent, 7bit is assumed. Quoted-printable suits mostly readable text with some special characters; base64 suits binary files and text in non-Latin scripts.
Choosing the encoding before signing matters: a server that converts 8-bit content for a 7-bit hop changes the bytes and breaks DKIM, which is why RFC 6376 advises encoding before signing. Quoted-printable or base64 also keeps HTML within the 998-character line limit.
Example
Content-Transfer-Encoding: quoted-printableRelated terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes