Jonathan B. Postel (November 1981). "4.5.3. SIZES". SIMPLE MAIL TRANSFER PROTOCOL. doi:10.17487/RFC0788. RFC 788. The maximum total length of a text line including the <CRLF> is 1000 characters (but not counting the leading dot duplicated for transparency).
N. Freed; N. Borenstein (November 1996). "Abstract". Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. doi:10.17487/RFC2045. RFC 2045. Multipurpose Internet Mail Extensions, or MIME, redefines the format of messages
