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.