Plesk email problem

I have a virtual server with six or seven applications - mainly joomla and tikiwiki. None of the applications have the email working properly. My applications should send emails when new customers register but no email gets through to me. But when I do register a customer and approve them via the back end facilities, the email of confirmation does seem to get through. So some aspects of email work and others do not. I'm pretty sure that none of the contact forms on tikiwiki or joomla are getting through to me aswell. I need the email problem sorted out for me. My emails for all applications are on the [login to view URL] email facility (as suggested by 1and1) - they are not set up via Plesk. The problem seems to be an intermitent and problematic link between my applications and my email sender on plesk. But that's what I think - I need an expert to sort it out for me.