Could this be something to do with your own ISP or e-mail provider?
Currently, the notifications I'm getting from PlaymoFriends come down almost immediately after someone has posted in a topic I'm subscribed to, so I don't think it has anything to do with the mail server at Dreamhost.
I had this problem occasionally when I was registered at
another forum and it usually came right by itself eventually. I had an even worse situation where the notifications stopped coming altogether - nothing I altered in my settings would fix it, so I eventually decided to change the e-mail account I had registered... and that solved the problem. I deduced from this that my previous ISP had been blocking the messages, though I have no idea why.
I hope it comes right for you soon, as I know how annoying it can be.