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SMTP Connection Timeout on Guest Linux OS to Remote Exchange Server


we are running ESXi 4 and have some VMs installed. One is a Linux (CentOS 5.3) and one is a Windows 2003 server.

The Linux machine cannot send email to a remote Exchange server. The Windows server has no problems.

When using telnet on the Linux machine to port 25 on the remote server, it connects, but hangs on the line: escape character is ].

After some Enters, the welcome message from Exchange mailserver is shown and the HELO command etc. can be entered.

The windows server connects immediately to the mailserver and displays the message.

Also when sending a mail to a recipient on the exchange server through EXIM commandline, this generates a timeout. (another linux server in the same rack, not a VM, can send mail and telnet correctly to the remote server.). I also tested another exhcange mailserver and this one also generates a timeout. One more exchange server is connection as it should.

I tried disabling SELinux and IPtables. Even tried bypassing the hardware firewall. Also I have installed another Linux distribution. None of these have the desired effect.

ReverseDNS is working OK. Hostnames can be resolved. This all works.

Does anybody have an idea what to do next?
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1 Solution
That sounds like classic interference.
What else is on the Exchange server? AV, Antispam?

Have you checked the network card drivers are up to date?

gaffieAuthor Commented:
Well, the mail servers (Exchange) are all three receiving mail perfectly. Just not from our Linux servers on ESXi. Also, the Linux server (non VM) behind the same firewall as the VM Linux is able to connect without any problems. The Windows server (VM) is also able to connect without problems.

I have added an image of the situation, maybe this will make it easier to understand the problem.

There are five locations in the image.
Location A: Our rackspace in the datacentre with two physical servers. One ESXi and one Linux. On the ESXi there are two VM's. One Linux and one Windows. These are behind the same firewall (cisco ASA).
Location B is another rackspace Linux server at another datacentre.
Location C is our own Exchange server at the office.
Location D and E are a clients with Exchange servers.

From location A telnet (or sending mail with Exim) fails from LinuxVM to Location D and E.
From location A telnet (or sending mail with Exim) is successful  from:
- LinuxVM to location B and C.
- WindowsVM to location B, C, D and E.

From location B telnet (or sending mail with Sendmail) is successful  from:
- Linux to location A, C, D and E

I hope this better explains the weird problem.

Have you got the SMTP scanning feature turned off in the ASA? I have seen that provide odd results more than once and turn it off by default.

gaffieAuthor Commented:
We have tried the smtp scanning feature, both on and off, without any success.
gaffieAuthor Commented:
Solved! The problem was the firewall. Both clients had a Zyxel Zywall 5 and apparently the firmware 4.00 has a bug that causes this error. Upgrading to the latest (4.04) firmware solved the problem. The other client also had a Zywall, but had already firmware 4.04 installed.
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