Indicates the encoding method used to represent binary or text data in an email message part. Possible encodings are:
- 7bit - up to 998 octets per line of the code range [1..127] with CR and LF (codes 13 and 10 respectively) only allowed to appear as part of a CRLF line ending. This is the default value.
- quoted-printable - used to encode arbitrary octet sequences into a form that satisfies the rules of 7bit. Designed to be efficient and mostly human readable when used for text data consisting primarily of US-ASCII characters but also containing byte values outside that range.
- base64 - used to encode arbitrary octet sequences into a form that satisfies the rules of 7bit. Has a fixed overhead and is intended for non text data and text that is not ASCII heavy.
The 8bit encoding is suitable for use with SMTP servers that support the 8BITMIME transport SMTP extension:
- 8bit - up to 998 octets per line with CR and LF (codes 13 and 10 respectively) only allowed to appear as part of a CRLF line ending.
The "binary" encoding is not generally usable with SMTP.
