Most forum newsletters are sent out automatically to registered members email addresses. However, many times an email is ignored. Have you considered sending the site newsletter to each member's private message box on forum as well as email?
I usually create a Monthly Newsletter thread and update it each month with a certain volume. I almost always make it in an image format so that it's eye catching. It's always posted in the announcements board.
Sending newsletter as a PM, I wonder... Newsletter is a media for forum to keep in touch with the not-so-active members, who don't visit a lot, or for those who haven't visited in a long time to know what happened with the forum. So if it's being ignored in e-mails, I don't think it'll be too different with PM, as only active members will check PM and they will already know what happens in the forum.
When I send newsletter I only send it to e-mails. I don't really post it at the forum, though it also can be done to help summarize the month and catch attention of guests.