Mailto is used to prepare an e-mail to be send by the user. You can parse mail, subject, cc, bcc, body but not everything will work the same way across mail clients and browsers. IE, for example, will skip your body part if you haven’t encoded it properly. Use %20 instead of space, %23 instead of # or just use a converter like CHA’s Mailto Code Generator. Also mind the character limit of approximately 2000, so don’t go too crazy.
Here is a lazy example:
<a href="mailto:firstname.lastname@example.org?subject=Title%20of%20message&body=%23hashtag%20lorem%20ipsum%20pound">Contact me</a>