I knew after I read Elle's post last night what your next one would be. I guessed right.
While dreamhost is a rockstar when it comes to webhosting, when it comes to email the star does not shine brightly. (and they admit it). Dreamhost email has gotten better in the past year, but dreamhost IP's do pop up on RBL's with some frequency.
If email delivery is critiical to your needs, you might want to split that off to a host that specializes in email. I found a solution that cost just $4/month and seems to be extremely reliable.
How does that work and still use dreamhost for web hosting? Just enter a custom MX record for your domain and a few TXT DNS records, wait for it to propagate and your mail is handled elsewhere.