The text in the mail message file starts with a number of lines that form the message header. These must be followed by at least one empty line to separate the header from the message body.
If RexxMail finds a default closing text file, its contents are added to the end of the message.
When the message file is opened, an attachments folder (the one with the paperclip logo background -- thanks again, Luc van Bogaert!) is also opened on the desktop. File or URL objects can be dropped in this folder to be attached to the message when it is sent.