If the message is in a different character format than the one you specify, or if transfer encoding is not specified, the message may become corrupted during data transport. See RFC 2045, Section 6.1, on the following Web site:
If an entity is the type “multipart” the content transfer encoding is not permitted to have any value other than 7bit, 8bit, or binary. See RFC 2045, Section 6.4, on the previous Web site.
The output of base64 encoding confirms to the charset US-ASCII. See RFC 2045, Section 6.2, on the previous Web site.
Valid values are 7bit, 8bit, binary, quoted-printable, base64, ietf-token, and x-token, and the parameter entries are not case-sensitive; not required.
Although this parameter is not required, it is recommended.