Content-Type
The MIME header that states what a message or part contains, such as text/plain; charset=utf-8, text/html or multipart/mixed, with parameters like the charset or the boundary.
What it means
A Content-Type value is a type and subtype, optionally followed by parameters (RFC 2045). Text parts carry a charset parameter; multipart types carry the boundary string that separates their parts; attachments carry a type such as application/pdf. When a message has no Content-Type, MIME says to treat it as text/plain in US-ASCII.
Mail programs decide how to show each part from its Content-Type. A wrong value causes visible trouble: HTML labelled text/plain shows raw tags, and a part with the wrong charset turns accented letters into garbage.
Example
Content-Type: multipart/alternative; boundary="b1_7f3a9c"Related 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