What causes an "javax.activation.UnsupportedDataTypeException: no object DCH for MIME type xxx/xxxx javax.mail.MessagingException: IOException while sending message;" to be sent and how do I fix this? [This happens for known MIME types like text/html.]
It doesn't know how to handle the text/html content type.
An html handler is provided in JavaMail 1.1.3 but the mailcap file is not configured to use it.
Add to your mailcap file the line:
and create a multipart message when sending html messages, otherwise it will not be recognised as an HTML page from the client.
HTML messages does not cause this problem with the 1.2 release of the JavaMail API.