You shouldn't use mailto: to send emails. For starters, it assumes the user has an email client installed on their device. If they haven't, then it won't work and they won't be able to send the email.
The info you fill in a form is exactly the same as what would be required in the email client, so there is no extra work or input for the user. Sending the email server side will work 100% of the time. Sending it client side will work in only some cases, so I will never use mailto: on a contact page.