i’m assuming you are using contact form 7 for wordpress… i’m having the exact same issue across the board on all of my contact form 7/wordpress installs. the only emails that seem to be delivered instantly are emails “from” a dreamhost hosted email address, if i fill out the form with a non dreamhost address then the emails are either massively delayed or not delivered at all. this is not a junk/spam issue and i’ve set up the exact same install on two other hosts to test and both of those installs are delivering the emails instantly. this seems like a dreamhost mail server issue.
i submitted a support ticket and i recommend you doing the same.