PlainText property will return part of the message that
has content-type of plain/text. It could be whole message,
but also any of its parts. wodSmtp will search through all
message parts until it finds text/plain part, and return it
through this property.
In specific occasions, if ContentType is
multipart/mixed, wodSmtp will go
one level deeper and check if any of parts have ContentType
of multipart/alternative - in which case text/plain content
will be searched among those parts. This is typical message
with two parts (plaintext and html) and attachment.
If plaintext content is encoded, wodSmtp will decode it
before it returns it to you through this property.