DNS/Outgoing Mail Help

I am having trouble with my mail settings for my website, which I believe has to do with my dns settings. For instance, when visitors sign up to forums on my site they do not receive registration email, newsletter issues don't arrive for subscribers with certain email addresses (i.e. [login to view URL]). I just need some help in resolving this issue. Someone to test it out, log into my WHM account if necessary and fix the problems. I have a Virtual Private Server and I may have not done everything right when setting it all up. I will supply you with whatever you need to resolve this issue. I would also like to be informed of what you did to fix the problem if it's not too complicated to explain it to me. This should be a fairly simple and easy project for someone who knows what they're doing. Thank you!