So, email forwarding is tricky, because if the messages you’re forwarding are spam — which will happen quite a bit if you’re forwarding all the mail sent to an address — it starts looking a lot like DreamHost is sending the spam, rather than just passing it on. During the time that we allowed email forwarding to these providers, they frequently ended up thinking our mail servers were a spam source and blocked us entirely, preventing mail from being sent to them at all (as well as making the forwards stop working).
An explanation from back when this policy was implemented is available at:
There’s a more detailed explanation of the situation written by a DreamHost customer at:
AOL and Comcast are the only providers we’ve had to do this for, though. Forwards to other providers should work fine.