Make sure your rDNS for your server matches the helo on your server. Generally your hostname resolves to an IP# which should resolve (rDNS) to the same hostname. If you've set it up in DirectAdmin then check to make sure it works, since most hosting companies don't let you set your own rDNS. If they don't, make sure they set it for you.
You may also need to set up DKIM for some providers to like your emails.
Make sure your server emails are coming from a valid return address.
Make sure you tell your new clients (on your signup page) to check their Spam box, and to also set up your return address in their address book.
Jeff