Not sure, but a few notes:
1) Bulk mail from the web machines isn't really encouraged, and some stricter restrictions on this may be in the works. If you're going to be sending out a lot of mail, please use the announcement list function in the web panel, or an outside server. How do we define "a lot"? Well bottom line, if it starts to cause problems for other users, you'll probably hear from us.
2) Make sure the program supports confirmed opt-in, as well as anything else required by our spam policy (http://dreamhost.com/spam.html)
3) Make sure that the program is configured to set the envelope-sender properly (many PHP programs don't do this by default) - if the program uses PHP's mail() function, this is controlled by the last argument (4th or 5th), which needs to be set to "-email@example.com", where firstname.lastname@example.org is the address you want bounces to go to. The program claims it does bounce tracking - you'll need to make sure that this is setup and working properly as well.
Side note: The envelope-sender shows up in the "Return-Path:" header of the email message you receive, and it's added by the receiving MTA with the value of the address sent during the MAIL FROM portion of the SMTP transaction. It's kind of like the return label on a snail mail envelope - if a letter were undeliverable, it would get returned to the return address on the back of the envelope - but if you were to reply, you'd probably write to the address at the top of the letter itself (like the "From:" header in the email). These two addresses might be the same, but they might not.
If you don't set the envelope-sender, and a PHP program is running as mod_php, it will get sent with an envelope address of "dhapache@[machine].dreamhost.com", which goes nowhere. If you are running it as php-cgi, it will get sent (by default) as username@[machine].dreamhost.com, which is at least valid, and will go to the corresponding mailbox.
I've seen a lot of people using Mojo mail, which is now called "dada".