I believe mail coming directly from a shared server or VPS is considered high risk by many spam filters. I got around this problem myself by create a dummy e-mail address on my account, and installing the "WP-Mail-SMTP" plugin to route outbound SMTP mail via dreamhosts mail servers. (in my case sub4.mail.dreamhost.com) It can be configured to send using Authentication and TLS to permit the outbound relay. Then you also gain the automatic DKIM signing when it passes through that mailserver.
I also encourage you you setup an SPF record on your domain to allow authentication of that server. at a minimum:
example.com TXT "v=spf1 include:netblocks.dreamhost.com -all"