[snakes1100]

You need the mail.log / mail_log depending the the servers OS, that is the only way to find out exactly why the email server it is sent to is putting them in the users junk/spam folder, there are numerous reasons this can happen, probably the most notorious one is the domain that the email originates from, sample:

forums@server.yoursite.com

As the server. part is in the email, its not a correctly formatted email from address.

Make sure your host has setup a valid PRT record for your IP as well.

There are to many reasons to list here.

Get the email log from the server and start checking the hosts that are marking your emails as spam, there are numerous ways to resolve this depending on your host and if this is a dedicated server, you may have to resort to using the smtp feature built into vbulletin to use a real outgoing valid email account.