Subject header
The header field holding the message's topic line, shown in the inbox next to the sender name and preview text.
What it means
RFC 5322 defines Subject as a short string identifying the topic, appearing at most once, and notes the convention of starting a reply's subject with Re:. Text outside ASCII is carried as an encoded-word. Gmail starts a new conversation when the subject changes and rejects a subject that is too long with 552 5.3.4.
Google's guidelines also ask that subjects represent the content accurately: not starting with Re: or Fwd: unless the message really is a reply or a forward, and not misleading in any other way.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322, section 3.6.5: Informational Fieldsrfc-editor.org
- Google, Gmail Help: Email sender guidelinessupport.google.com
- Google, Gmail Help: Group emails into conversationssupport.google.com
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes