In the .eml format? Why not just save a html file (with the html) or text file (with the plain text) + the attachments in a folder. Also, the e-mails can be accessed directly from the email template module. For history references, this includes attachments.
If you want to send the e-mail to another e-mail you use the BCC or CC options. Or you send another e-mail based on a template with the attachments + html/text file attached to that e-mail address.
I don't see why you would want to generate some sort of e-mail file just for the sake of archiving or sending it as a copy. Unless the requirement is getting it as an .eml file. (.eml is BTW the format Microsoft Outlook uses to save e-mails externally)
See here on stackoverflow: http://stackoverflow.com/questions/157195/create-a-eml-email-file-in-java