Internet E-Mail (RFC-822)
We all know and use it, but here is a formal specification.
Each message is a stream of 7-bit ASCII chars which contains a header and optional (newline separated) body.
Header consists of a set of entries with one entry per line given by a colon separated key:value pair.
Key contains no spaces or tabs and cannot exceed 63 chars.
Body is a fully unstructured sequence of ASCII chars.
There is a finite set of standard keys and an extension mechanism via the "X"-prefix. The standard set (as used by MH) is:
Date Bcc Resent-Date Resent-Fcc
From Fcc Resent-From resent-
Sender Message-ID Resent-To Message-Id
To Subject Resent-cc Forwarded
cc In-Reply-To Resent-Bcc Replied