You need to manually add the new lines to the message. Try the following:
... StringBuffer sb = new StringBuffer(); sb.append("This is line 1 "); sb.append("This is the second line "); sb.append("..."); msg.setText(sb.toString()); ...
You must use one body part with text but the text can contain multiple lines. So you need to use the.
You can also use
String newline = System.getProperty(“line.separator”);
to get the platform-specific line separator. Keep in mind that if you display the text as HTML, the newline is just white space.