Encoded-word
The RFC 2047 syntax for putting non-ASCII text into email headers, written =?charset?encoding?text?=, used for subjects and display names with accents or other scripts.
What it means
Header fields must be ASCII unless SMTPUTF8 is in use, so a subject or display name with other characters is encoded: =?UTF-8?B?...?= for base64 or =?UTF-8?Q?...?= for a variant of quoted-printable. An encoded-word may be at most 75 characters long, so longer text is split into several separated by folding white space, and mail programs decode and join them for display.
Encoded-words are allowed only in certain places: unstructured text such as Subject, comments, and words of a phrase such as a display name, never inside an address. Gmail rejects headers that put an encoded word where it is not allowed, and also lists a raw Unicode character in a header among its rejection reasons.
Example
Subject: =?UTF-8?Q?Votre_facture_d=C3=A9taill=C3=A9e?=Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2047: MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Textrfc-editor.org
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
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