SMTP port open - but not open

Posted by Frederik Nielsen on Server Fault See other posts from Server Fault or by Frederik Nielsen
Published on 2012-07-04T08:49:25Z Indexed on 2012/07/04 9:17 UTC
Read the original article Hit count: 381

Filed under:

firewall

|

smtp

|

exchange-2010

|

port-forwarding

|

packet

As some of you might know, I am setting up an exchange server.

Now I ran into another problem:
I cannot connect to the SMTP service from outside the server!

The ports are opened in the gateway device (a ZyXEL USG50), Windows firewall is off.

I see the packets travekl through the ZyXEL firewall, and I can also see the packets with wireshark on the server, so I know they are getting all the way in to the server.

I also know it receives them, and sends out the reply - and this is where things go bad!

Analyzing with wireshark, I get these errors in the return packets:

Header checksum: 0x0000 [incorrect, should be 0x0779 (may be caused by "IP checksum offload"?)]
And:
Acknowledgment Number: 0x8e3337d1 [should be 0x00000000 because ACK flag is not set]

What the (sorry my French) hell is going on?
I really cant figure it out..

Thanks in advance.

© Server Fault or respective owner

Related posts about firewall

Managed hosting firewall vs managing own firewall

as seen on Server Fault - Search for 'Server Fault'
I posted on stackoverflow as to the overall benefits of managed hosting vs non-managed hosting. The more I think about it, it seems to boil down to one question: should I use a managed host because they take care of the firewall, or would I be okay managing my own, software firewall? The sites… >>> More
Hardware firewall vs VMWare firewall appliance

as seen on Server Fault - Search for 'Server Fault'
We have a debate in our office going on whether it's necessary to get a hardware firewall or set up a virtual one on our VMWare cluster. Our environment consists of 3 server nodes (16 cores w/ 64 GB RAM each) over 2x 1 GB switches w/ an iSCSI shared storage array. Assuming that we would be dedicating… >>> More
Firewall behind firewall

as seen on Server Fault - Search for 'Server Fault'
I've recently changed jobs and I've been set up with a new workstation. On all previous places where I've been working they've had some sort of local firewall installed on each and every workstation - but here I've been told not to activate it because it is not necessary since we're already behind… >>> More
We have no SW Firewall behind our office HW firewall, admin says its not req'd

as seen on Server Fault - Search for 'Server Fault'
I've recently changed jobs and I've been set up with a new workstation. On all previous places where I've been working they've had some sort of local firewall installed on each and every workstation - but here I've been told not to activate it because it is not necessary since we're already behind… >>> More
Can't get FTP to work on centOS 5.6

as seen on Server Fault - Search for 'Server Fault'
Hi guys I have been trying for a few hours to install and get FTP to work... I did yum install ftp and yum install vsftpd They all installed and are running but when I try to use filezilla or some other client I just can't connect....I've tried connecting on port 21 and port 990 ....nothing! These… >>> More

Related posts about smtp

JavaMail not sending Subject or From under jetty:run-war

as seen on Stack Overflow - Search for 'Stack Overflow'
Has anyone seen JavaMail not sending proper MimeMessages to an SMTP server, depending on how the JVM in started? At the end of the day, I can't send JavaMail SMTP messages with Subject: or From: fields, and it appears other headers are missing, only when running the app as a war. The web project… >>> More
Gmail smtp, Blackberry, BB, socketConnection, send email via gmail smtp programmatically

as seen on Stack Overflow - Search for 'Stack Overflow'
Hello! Somebody can help me how can i use the gmail smtp server programmatically via socketConnection. My question is how i can write write a TSL/SSL authentication because i can`t communicate with the server?? Somebody did it from java on blackberry ? Thank You Alex >>> More
How to resovle javax.mail.AuthenticationFailedException issue?

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi, I am doing a sendMail Servlet with JavaMail. I have javax.mail.AuthenticationFailedException on my output. Can anyone please help me out? Thanks. sendMailServlet code: try { String host = "smtp.gmail.com"; String from = "[email protected]"; String pass = "pass"; Properties… >>> More
Debugging Messaging Exception

as seen on Stack Overflow - Search for 'Stack Overflow'
We have a batch program that incorporates JavaMail 1.2 that sends emails. In our development environment, we haven't got the chance to encounter the above mentioned exception. But in the client's environment, they had experienced this a lot of times with the following error trace: javax.mail.MessagingException:… >>> More
Postfix Submission port issue

as seen on Server Fault - Search for 'Server Fault'
I have setup postfix+mailman on my debian server and i have an issue with postfix submission port. My ISP blocks SMTP on port 25 to prevent *spams and i must to use submission port (587). I have uncomment the following line from master.cf (/etc/postfix/) but nothing happens. submission inet… >>> More