Multiple domains having email sent by the same server

Here's the scenario:  We own four different companies and have four independent domains with the email server for each domain hosted on the same machine (same physical machine, same physical software, same static ip address).  In the past, we've had issues with email sent from this server being blocked because of no reverse DNS on the domains so we had that set up a few years ago.  For the most part, this works fine, but we still run into a few servers rejecting our email.

Since all four domains have a reverse DNS pointing to the same IP address, when you do a reverse DNS on the IP address, you will have a different one of the four domains returned as the hosting domain each time.  By this I mean on the first reverse DNS lookup, you will be told that domain #1 is the hosting domain.  On the second reverse DNS lookup, you will be told that domain #2 is the hosting domain and so on for the 3rd and 4th lookup.  On the 5th lookup, it rolls back to listing domain #1 as the hosting domain and repeats the whole thing in the same order.  By doing this, I would think that you actually only have a 25% chance of the domain that the email is being sent from actually being the domain that is returned when the receiving email server does a reverse DNS to check to see if it's spam.

My questions are:
1. How should I set things up in this situation so that the reverse DNS functions properly?
2. We're using Microsoft Echange Server as our email server so could we do something like assign more static ip addresses to our DSL connection and then use those other ip addresses with new virtual SMTP servers in Exchange?  If so, how would that work with only one NIC installed in the server?
3. How would you tell Exchange Server which ip address to use to send the email for each domain?

Thank You in advance for any help available on this issue.
crosstecAsked:
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jar3817Commented:
You just need 1 PTR record. It doesn't matter what it is, as long as it is descriptive and non-generic (mail.oneofyourdomains.com). The reverse dns doesn't have to match the FROM domain on the email, so you don't need to worry about the other 3 domains, just use whatever the main domain is.
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crosstecAuthor Commented:
I received a new email about a message that was blocked today.  In the response message, it says "Client host [mail.servername.com] blocked by zen.spamhaus.org; http://www.spamhaus.org/query/bl?ip=<insert ip address here>"  The internal static ip address for one of our servers that sends shipment confirmation emails was in the <insert ip address here> spot.  Of course, the internal ip address is different than our external ip address, but how is this internal ip address being sent along with an outgoing email?  Shouldn't the internet only be able to see our external static ip address?  When following the link in the email, it states that the ip address range has been identified by Spamhaus as not meeting their policy for IPs which should deliver 'direct-to-mx' mail to PBL users.  That's completely understandable, as this ip address shouldn't be seen in the first place.  Spamhaus should only be seeing our public ip address, right?  How do we fix this?
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jar3817Commented:
Does this server that sends the shipment confirmations relay all outgoing mail to your main mail server? Perhaps that is the server that checks spamhaus? That's the only way I can think of for the internal IP to be checked.
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crosstecAuthor Commented:
The server that sends the shipment confirmations does relay the mail through the main mail server.  It doesn't contact spamhaus in any way, though.  It's just an internal server that uses the Exchange Server on our main server to send these emails.
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