The Content-Transfer-Encoding header specifies an auxiliary encoding
that was applied to the content in order to allow it to pass through
mail transport mechanisms (such as SMTP) which may have limitations
in the byte ranges that it accepts. For example, many SMTP servers
do not accept data outside of the 7-bit ASCII range and so sending
binary attachments or even non-English text is not possible without
applying an encoding such as base64 or quoted-printable.