The type of encoding to be used in the email headers.
This value can only be 1 of 4 types (according to RFC2047):
By default, this value is MailEncoding.QuotedPrintable, and is only used if the CharSet is not US-ASCII.
If MailEncoding.Bit7 or MailEncoding.Bit8, then all encoding is ignored, and the mail is sent as is, without any encoding. This may have undesirable consequences, if the header has 8 bit characters, which some mail gateways, or clients, may not be able to interpret. This flexibility will allow the user to perform their own encodings, that my be proprietary to their systems.