I have a DSL line and purchased a block of addresses so that I can run my own DNS server, web server, mail server, etc. I operate several domains within this IP block, maintaining DNS and reverse DNS entries on my name server.

Lately I have found that some of my out-going eMails are being blocked because when the receiver's mail server does a reverse DNS lookup of my server's IP address, they get a Sprint DSL domain name, not the domain name I have registered. For example, if your run a "dig" command against my domain www.VorlonInfoTech.com you can find the MX record of 65.40.143.219. However, if you do a reverse DNS of this same address, you get "user219.net458.oh.sprint-hsd.net".

Many large ISP's use commercial spam filters (such as www.five-ten-sg.com) which will drop any message where the domain name does not match the reverse DNS domain.

I haven't contacted Sprint to try to get their reverse DNS entry removed yet, but suspect that they will be hesitant to remove their entry. And if they do, how can I get the world to look at my server to find the reverse DNS entry?


Any suggestions for how to fix this problem (which will probably only get worse as time goes by) would be very much appreciated.


Casey Bralla
Chief Nerd in Residence
The NerdWorld Organisation

i'm not sure but since the ISP owns the IP number
you will need their cooperation and they will have to change
rDNS lookup
hopefully they will do that ??

I've been running an email server for about a year and a half. When I first set it up I was having configuration problems and I wondered if this was the issue. I have heard that some large ISPs do this. I have also heard that some simply check to ensure a reverse DNS entry exists, but do not necessarily need it to match your domain.

In my opinion, this approach by large ISPs is stepping on the little guy. Their goal is to prevent spam, but the result is to shut out small legitimate email servers. Having said that, I have not bumped into this as a major problem. In fact, I do not think it has affected me at all at my server.

I have heard that some ISP will accommodate you with reverse DNS. Others will not.

Yeah, this actually did affect me. I had to ask my ISP to setup PTR records (which is techno-speak for reverse DNS) so that the reverse DNS lookup matched the domain of my mail server. For example, if my mail server is mail.yourdomain.com, you need your PTR records to say the same thing. This is the absolute best way to have it setup. This way, everything matches perfectly and there should be no problems.

Yes, if your ISP will make Reverse DNS (PTR record) changes for you, then that is the best way to do it. Some times your ISP won't do that (too much work for them) or some times stupid companies subscribe to RBLs (Realtime Blackhole Lists) that block any IPs from dynamic assignment blocks, or even any IPs from broadband networks.

In that case, there's not much you can do except call up the company you're trying to send mail to and plead with them to use some common sense and dictch their error-porne methods of blocking spam. RDNS for example has some very fundamental problems, like what happens if one server hosts more than one e-mail domain? That's a perfectly legitimate e-mail setup, but most RDNS implementations will block all the mail that doesn't match the domain from the PTR record.

There are much much more accurate ways to block spam (although most of them cost money). The current generation of commercial anti-spam technology is very good if you purchase from the top vendors (or very poor if you purchase from one of the low-tier "me too" vendors).

If all else fails, ask the administrator of the domain you're sending to for a whitelist entry that will allow your e-mail to bypass their normal filters.