Replace usage of javamail with some custom code

At the moment we're using javamail to output mime multipart/mixed messages. This is extremely inefficient, since javamail makes the output formats runs twice in order to decide wheter each part of the message can be encoded as 7bit or base64 (it scans the output of the first run to decide wheter the content is made only of ascii chars or not)

Never mind, if one forces the content type encoding to base64 before write occurrs the guess code won't run