If POP3ClientSession::retrieveMessage is used to retrieve a plain/text email message. It seems the MailMessage::_encoding is not being set.
Resulting in MailMessage::write not being able to find the right encoder, thus not printing the content to the stream.

I think there are two solutions, either to set the MailMessage::_encoding in MailMessage::readPart
Or have a default StreamCopier in MailMessage::writeEncoded

My version is: POCO 1.6.0-all (2014-12-22)