I think they have good support. BUT!! But mailto will be used to send and send email from the client TO me@somewhere.com. That is definitely you are not sending out the newsletter.
You will have to send emails from the server, possibly in the background.
EDIT: Wrong rethinking. Are you saying that you are using the admin section with a link to several TO addresses that the email client will then open to send your newsletter? In this case, despite the fact that large browsers probably have decent support, you should not rely on it in the wild, since it is not in the standard (I think), so they are not required to support it.
EDIT AGAIN:
rfc 2368 info in support of my previous edit
Dave archer
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