Mail throttling add-on to Mail_Queue?

I was recently directed to the PEAR:Mail system by the PHP manual as I wanted to support HTML mail on my site. Looks fairly interesting, but as with a lot of tools like this, sometimes the documentation doesn't quite go far enough.

Anyway, my question is this: I am intrigued by the Mail_Queue package. It appears that the goal of Mail_Queue is to create an efficient way to send larger volumes of mail versus the PHP mail system which is meant for onesie-twosie types of mailings.

My interest in Mail_Queue is because of the following: many webhost providers have an "emails per hour" limit to prevent spammers from using their systems. I am not a spammer and I support this concept, yet, my site has grown significantly and as I add more message board and private message capabilities to my site, my outgoing e-mail volume has grown. Usage on my site is very cyclic and I don't need a higher mail volume all the time, so my thought is to put some kind of throttling system in place that would delay sending low priority e-mails out until such time as it would not put me over my webhost provider's limit. I have to concept of "priority" on my site, as password resets and e-mail address verification e-mails should take precedence over message board post e-mails. I thought Mail_Queue might be able to do this, however, it does appear to stop quite short.

However, I noticed that as-is, it does appear to support the necessary data structures to build such a system on top of it, namely it appears to keep track of when e-mail is finally sent and it does support a field, "time_to_send" that could possibly be used as a priority indicator.

My question is, has anyone heard of such an add-on? And if not, is there anyone that has more details on some of the parameters to the Mail_Queue package? Specifically details on how the sec_to_send parameter is treated by the various package routines?

Thanks,