Create a .eml (email) file in Java

EML files are just plain text files. The headers are separated from the body by a blank line. Headers look like this:

From: "DR CLEMENT OKON" <[email protected]>
To: "You" <[email protected]>
Subject: REQUEST FOR URGENT BUSINESS RELATIONSHIP 
Date: Tue, 30 Sep 2008 09:42:47 -0400

For more info, the official spec is RFC 2822. It's actually not as hard to read as some RFCs.

Edit: When I said "plain text" I should have thought for a second. I really meant plain ASCII - and not the 8-bit "extended ASCII" either - just up to character 127. If you want more than seven bits, you need some kind of encoding and things get complicated.

You can create eml files with the following code. It works fine with thunderbird and probably with other email clients:

public static void createMessage(String to, String from, String subject, String body, List<File> attachments) {
    try {
        Message message = new MimeMessage(Session.getInstance(System.getProperties()));
        message.setFrom(new InternetAddress(from));
        message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to));
        message.setSubject(subject);
        // create the message part 
        MimeBodyPart content = new MimeBodyPart();
        // fill message
        content.setText(body);
        Multipart multipart = new MimeMultipart();
        multipart.addBodyPart(content);
        // add attachments
        for(File file : attachments) {
            MimeBodyPart attachment = new MimeBodyPart();
            DataSource source = new FileDataSource(file);
            attachment.setDataHandler(new DataHandler(source));
            attachment.setFileName(file.getName());
            multipart.addBodyPart(attachment);
        }
        // integration
        message.setContent(multipart);
        // store file
        message.writeTo(new FileOutputStream(new File("c:/mail.eml")));
    } catch (MessagingException ex) {
        Logger.getLogger(Mailkit.class.getName()).log(Level.SEVERE, null, ex);
    } catch (IOException ex) {
        Logger.getLogger(Mailkit.class.getName()).log(Level.SEVERE, null, ex);
    }
}

Looking at a typical EML file it looks like a raw dump of the text communication that went to the server. So it is a text file containing the mail headers and body. To get your attachments, different views, etc in the correct format inside the EML file you need to MIME-encode the body and its parts.