The best way to write a message is to use the URL keyword. For example if you want to write an email like this:
TO: someone@example.com,anotherone@example.com
CC: someoneelse@example.com
BCC: andsomeoneelse@example.com
Subject: Summer Party
Body:You are invited to a big summer party!
You can write:
launch url "mailto:someone@example.com,anotherone@example.com?cc=someoneelse@example.com&bcc=andsomeoneelse@example.com&subject=Summer%20Party&body=You%20are%20invited%20to%20a%20big%20summer%20party!"
Remember to urlencode the subject and the object.