Be wary of how many and how often you send mail. If you're using a shared server and your host sees that your site alone is using an exorbent amount of CPU and memory, they'll cut you off. So even using other packages may not help decrease CPU utilization if you're sending 5k messages; however, it will be less than using mail().
If you're doing it as an online type of mailer, make sure that your script is "SPAM PROOF". Don't allow hackers in, and don't give the easy access. Make those people that are sending out the emails provide their email logins (address & password & visual confirmation). Check all of those, if they're fine, send the newsletter from that address, not your own (just for responsibility purposes).
Do validation on any and all data received from forms. Worst thing you could do is allow a hacker an easy target without using mysql_real_escape_string() or allowing any and all HTML. Just be weary.
With HTML mail you have to think about Java & viruses. If you don't want to allow java, remember to do a check to make sure that the message body doesn't contain:
<script lang="Javascript" type="text/java"></script>
tags.
Use some common sense. Look around the internet on what is going on: Email viruses, injection of code, illegal attachements... a lot from unsuspecting people. Just be careful, and if you run into problems when you don't know how to validate data, or you can't figure out how to fix an injection problem, ask the community.
~Brett
