Email is definitely not one of DH’s strong points from what I’ve read. (I don’t know personally because I don’t rely on DH for mail). In some ways, the technical logistics of email hosting are more difficult than web hosting.
Tons of SPAM needs to be filtered. This is a very delicate balancing act.
Messages need to be stored and resent if the reviving server goes down.
Because some SPAM gets through, severs can get loaded down with traffic.
Multiple copies of the same messages need to be stored along the way in case of problems.
And those are just the main hurtles I can think of off the top of my head. I’m sure there are more.
Bottom line, DreamHost is an amazing webhost – but email is not their specialty. I suspect that is the reason new accounts default to using Google’s mail service. If you like the web service, maybe you should consider Google’s (free in most cases) email service?