The Part property returns the current message part index.
All messages have at least one part, which consists of one or more
header fields, followed by the body of the message. The default
part, part 0, refers to the main message header and body. If the
message contains multiple parts (as with a message that contains
one or more attached files), the Part property can be set to
refer to that specific part of the message.
For example, messages with file attachments typically consist of
a message part which describes the contents of the attachment,
followed by the attachment itself. For a message with one attached
file, there would be a total of three parts. Part 0 would refer to
the main message part, which contains the headers such as From, To,
Subject, Date and so on. For multipart messages, part 0 typically
does not have a message body, since any text is usually created as
a separate part (for those messages that do not contain multiple
parts, the part 0 body contains the text message). Part 1 would
contain the text describing the attachment, and part 2 would
contain the attachment itself. If the attached file is binary, then
the transfer encoding type would usually be base64.