Sending tons of emails to certain users – options

I’m hoping to get some advice here; and maybe avoid problems in the future.

We post maybe 20 posts per day. Various criteria (language, location, etc) in a post are matched against our user database and for each post we extract a large list of emails of users who have opted to receive a post like that; that list is taken from a database call.

$results = $wpdb->get_results( $defsql );

This often returns several hundred email addresses per post.

We then simply email those users the basics of the post (with a link) using

sleep(0.5);
wp_mail( $user->user_email, $pTitle, $pContent, $headers );

We’re starting to send out more and more emails like this each day and I have a feeling that it’s probably not the best way to do things.

First off, does anyone know of a plugin which might help here? Or if not, are we heading for trouble since each day we’re sending out more and more (opt in) emails and I wouldn’t like to overload the system! Is there a downside to using wp_mail so much like this?

I don’t really know enough about the heart of WP and wouldn’t like to risk something here as the site is growing rapidly.

PS It’s our own server.