We have big problem with the email send from our dedicated server.

We have a dedicated server with iWeb and host many site on it.
The problem is that the email send from those site (example when user fill a form) are not always received depending of the email it send to.

iWeb told us that the server got blocked by hotmail, yahoo and maybe other.

What should we do?
How do we get delisted and back clean?
Is there a place to excatly know where we are listed? I have a site for the blacklist but is there one that tell us for yahoo, hotmail, ...?

And what must we do and don't do to not have this problem again?

Thank you