To massively increase deliverability and lessen the chances of your messages going to the spam/junk folder, ask your web host to enable all of the following on your domain and/or its IP address:
... as these are the facilities ISPs use to check if an email is spam.
Establishing a good sender reputation by warming up your IP address will also help. For more information on how to warm up your IP address, visit the arpReach online manual.