I recommend not forwarding email because it is likely you will have problems spam-filtering the forwarded email.
I had problems when I switched the MX record of my Dreamhost domain so, to me, it is scary to switch an MX record at Dreamhost. I've switched MX records at other providers with no problems, i.e., no lost or bounced mail. At Dreamhost they stop accepting mail for a domain as soon as you change the MX record. At all my other providers there was a period of time during which both the new and old MX host accepted mail for my domain.
Maybe this has now been fixed now at Dreamhost?
I've done a lot of research about this and documented it here:
I'm mainly using http://www.tuffmail.com/ and I'm happy with them. If it doesn't seem like they do what you're looking for, I recommend that you email them and ask. I know they do customized systems for some of their customers.
Good luck with all this!
Infinite Ink ~ http://www.ii.com
Deflexion & Reflexion ~ http://deflexion.com