Dreamhost has been having sendmail problems for many many months now. I can give you the same advice tech support gave me, which is working really well for me… rather than using sendmail, use SMTP… (use your mail.yourdomain.com server). My sites use PHP, so I was able to find a PEAR package that will do it, and something similar may work for you. The thing is, when you use sendmail, you’re sending all your email, unauthenticated, to like 1 or 2 outgoing servers, which is a huge bottleneck if other sites are doing the same. If you use your SMTP server, it will have less traffic, and your email will make it out quickly (and be authenticated).
I’m still converting over to SMTP on my sites, and it’s interesting to see… email sent using SMTP delivers promptly, while email sent using sendmail takes up to a couple days, as you pointed out. Hope this helps… and isn’t too much trouble to get your site converted.