Which email header field contains the message's title line?

The Subject email header field contains the message’s title line or brief topic summary.

In the Internet message format, an email is divided into a header section and a body. The header contains structured fields such as From, To, Date, and Subject. The Subject field gives readers a concise description of what the message concerns, and email applications normally display it prominently in inboxes and conversation views.

Subject is not the same as the sender or delivery history. The Sender field identifies an entity sending on behalf of the author when that differs from the From field. Received fields record the servers through which the message passed, while Priority is not the standard field that supplies the message title. Common subject prefixes include “Re:” for a reply and “Fwd:” or “FW:” for a forwarded message.

The Subject field is metadata rather than the message body. It can contain non-ASCII characters through MIME encoded-word mechanisms, allowing languages and symbols beyond the original ASCII-focused email format. The field is also separate from the visible content of an HTML email: changing the body does not automatically change the subject. Modern email clients may group messages using subject and other conversation metadata, but the header field itself remains the message’s declared topic.