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How to have multiple IPs handled by a single computer to support Email Server?

We have multiple domains that we want to send email from.  This means that we need to be able to have Reverse DNS set up for each of these domains to email delivered to some recipients (i.e. at AOL).  My current understanding is that I need to have a unique IP address for each domain for reverse DNS to work (that's what our ISP has told us).

Right now we support about 30 domains on a single server computer.  Probably we will only have about 5-6 domains that we will want to send email from.

We have a range of IP addresses from our ISP. (As I write this, it occurs to me to wonder whether we just need to have the PTR record set by our ISP to make AOL happy, or do they actually check what is going on at the IP address?)  However, how do we set it up so that we can have our single server handle multiple IP addresses?  (Our ISP said something about getting a router with virtual ports).
2 Solutions
If your mailserver's OS supports it, just bind multiple addresses to the public interface. Most mailservers can be configured to bind to a specific address for each instance. You're very vague on your environment, so its difficult to be specific.
hzukerAuthor Commented:
I don't think I was vague about the environment.  I didn't mention it at all.  ;->  We are running all MS stuff.  Specifically we are running Windows 2003 and IIS.
I don't think you need multiple ips. You can set multiple MX records to point to a single A record. and yes, PTR records are very necessary..
Check your DNS setup here before you get too far down this road:
http://www.dnsreport.com  just enter your domain names and see what it says.
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i agree with Lrmoore about you can use just one IP address with a properly set up DNS/MX records... i assume you are running exchange?  which version? since you didn't say we have to guess
You could just use one IP and map requests by name - but i dont know how :\
It would have to be done on your gateway/router...

and should be possible with one ip
hzukerAuthor Commented:
Excuse my ignorance in this area as I've just been shoved into the role of administrator here at my client's office, normally I code.  But from what I understand of the configuration here is that they use the default SMTP Virtual Server in IIS (which I didn't know existed before I got here - so I don't know the capabilities of it quite yet).  They send out a lot of outgoing email to customers.

I would prefer not to have to map multiple IPs if I don't have to.  Mapping MX records to A records sounds a lot easier and more practical if that works.

So if I understand correctly, if I have www.mydomain.com with an 'A' record that points to and it has a 'PTR' record, then I should  set up a 'MX' record mail.otherdomain.com to point to www.mydomain.com.  Hmmm.   I don't quite get this because if I do a reverse lookup on wouldn't it still return mydomain.com?  Or maybe it doesn't matter, just as long as a 'PTR' record is found.

You can setup your DNS:

yourdomain.com MX 10 mail.yourdomain.com
theirdomain.com MX 10 mail.yourdomain.com
anotherdomain.com MX 10 mail.yourdomain.com

server.yourdomain.com A  <ip address>
<ip address>  PTR.yourdomain.com

Personally I think having to reverse DNS on your domains is over the top unless you're a large corporation - I've never had problems with mail being rejected because my mailserver IP address doesn't resolve to my domain (at least with ISPs here in the UK).

You also mention that your ISP has recommended a router, so I take it that you connect to the internet with a modem? If you do I would certainly recommend a router and use NAT as a basic measure to protect your network - a firewall would be better. Virtual ports is just a term to forward a TCP port from the public IP address to a private one on your network so that external users can access services such as SMTP or HTTP.

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