mail() is slow

I have a script that regularly sends out lots of e-mails. Like, up to 500 at once. And since I don't want to use BCC or have everyone's address in the To: field, I've been calling up the e-mail list and looping through it, using one mail() command at a time. This has proven to be very slow. Is there a more correct way to do this? I checked out some other code I've messed with, like phpbb, and they just use Bcc and "undisclosed recipients" in the To: field. I don't want to do that, it'll make the e-mails more likely to get sent to people's spam folder.

Any other recommendations on the best way to deal with this?