Outgoing emails fail to certain recipients, but not to others.  Suspect Cisco PIX configuration is not allowing reverse lookups.

Posted on 2006-10-26
Medium Priority
Last Modified: 2013-11-16
Firstly, a little about my system.  I am using MS Small Business Server 2003, with f-secure proofpoint email gateway (it is fantastic!!!)and a Cisco PIX firewall.  The proofpoing gateway is a recent addition and the problems started at the point that it was installed, and it is reasonable to point a finger either to that, or the changes made to the rest of the system in order to accommodate it - such as changes to the PIX.

Now the problem.  I'm having some problems sending mail to certain addresses, where we get non-delivery receipts, often with custom worded error messages such as "sorry we don't accept mail from spammers" or "This system has been configured to reject your mail".  I'm fairly sure that the problem is to do with certain mail systems performing reverse lookups, which are failing, but I'm a little unsure quite what to do about it.
Looking at a NDR received today, the sending address is but our incoming mail is sent to; is this a common practice?
If I do a reverse lookup on, mail.prospec.co.uk is returned, but if I do a reverse lookup on I get the response that there are no PTR records for this domain, which is understandable as this is the outgoing address.
I think it is a problem with the way in which the PIX is configured - it is not letting reverse lookups in.
It is important to note that I have made some recent changes to the PIX, and it is since those changes that the problems have started.  To validate this, I have successfully sent a mail using the original PIX configuration, to a recipient that we get NDRs from when using the new PIX configuration, which proves that the problem is either a. a problem with the PIX configuration or b. a problem with the email gateway (f-secure proofpoint appliance), although I suspect it is the PIX.
The changes that I made were to accomodate the proofpoint appliance, where instead of all incoming traffic going to (original configuration), I now filter smtp traffic and forward that to (proofpoint appliance), and forward http to
I guess that now that I am being specific about certain types of traffic, I must specifically allow reverse lookups in, but I haven't a clue where to start and can't find anything on the web.
The original PIX config used the following command to route traffic to the server "static (inside,outside) netmask 0 0 ", but the changes that I have made routes smtp traffice to the email gateway and http to the server as follows "static (inside,outside) tcp smtp smtp netmask 0 0
static (inside,outside) tcp http http netmask 0 0
static (inside,outside) tcp https https netmask 0 0 "

But, I don't understand why the reverse lookups worked in the first place - to - because the only changes that I made were to the static commands, which did not include, but they did.
I look forward to a resolution to this problem, which may, or may not, be a problem with the PIX configuration - I may have got it badly wrong as this is not an area that I confess to being very knowledgable about.


Question by:CraigStenton
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LVL 79

Expert Comment

ID: 17810819
Could be a combination of things. Your email server is sending outbound email with one source IP and the MX record for inbound email is a different IP? This is a potential for confusion of upstream mail systems that look you up before accepting mail from you.

You need to make sure the outbound traffic of the email server matches the PTR and MX records. Since you changed from a 1-1 static nat to a port-forwarding pat, you might want to consider a conditional nat statement (need 6.3x or higher)

 access-list email_nat permit ip any eq smtp
 global (outside) 10
 nat (inside) 10 access-list email_nat

This is the only way to assure that outbound email from the internal mail server goes out with the same IP address that is used for inbound email.

You may need to adjut the fixup dns on your PIX depending on what version OS.
If you're running 7.x on the PIX, you may need to disable fixup esmtp

Author Comment

ID: 17812424
Thank you for your response.  I think I ought to tell you that I perhaps should have rated myself as a newbie/beginner on this subject, particularly with Cisco PIX.

I have looked at your suggestion, which seems to make a lot of sense, although I suspect there was probably a good reason why different addresses were used for incoming and outgoing transmission.
The first line of your suggestion sets up an ACL, but the PIX doesn't accept it, and I wonder whether there is something missing.  The way that I am working is to clear the configuration, modify the configuration in a text file and use the "configure net" command to upload the new configuration.  When I do this I get the following error:

ERROR: invalid IP address any
Config Error -- access-list email_nat permit ip any eq smtp
Global will be Port Address Translated
.ERROR: access-list <email_nat> does not exist

I would appreciate your further input
LVL 79

Expert Comment

ID: 17812449
My error in the syntax... try this:

  access-list email_nat permit tcp any eq smtp
                                            ^^  "tcp" vs "ip" makes all the difference...

>The original PIX config used the following command to route traffic to the server "static (inside,outside) netmask 0 0
The original config guaranteed the same inbound and outbound IP address for email.
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Author Comment

ID: 17813189
Thank you for your revised solution.  However, it still doesn't work as shown below:

ERROR: invalid IP address any
Config Error -- access-list email_nat permit tcp any eq smtp

Like I said, I'm a newbie, especially when it comes to Cisco PIX and need some guidance on this.

I look forward to your response.
LVL 79

Accepted Solution

lrmoore earned 2000 total points
ID: 17813280
Damn. Need more coffee... or maybe too much already..

 One more time. I tested it this time..

   access-list email_net permit tcp host any eq smtp


Author Comment

ID: 17841072
Just to let you know there is still a problem implementing your solution, which I am reading about to ensure that I understand it properly.  I believe that your solution is probably syntactically correct, and the problems are more to do with working it into my situation, where I have some existing conflicting commands.
Please bear with me - I haven't fallen off the face of the earth :->  - I'm just doing lots of reading (fitting it around many other tasks), which is why it is taking a long time to reply.

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