Today i’ve encountered a very interesting problem that’s very hard to track down exactly.
A small business customer was running an Exchange 2003 server behind a ZyXEL ZyWALL 5 with AntiSpam installed and enabled. The ZyWALL forwarded port 25 to the Exchange server. This worked, for the most, flawlessly. But a few hosts (i’ve found no distinct differences between the source hosts – ADSL, Leased Lines, Colocated, Europe, USA) failed to get an SMTP greeting (220 customer.example.com Microsoft ESMTP MAIL Service, Version: 6.0.xx ready at Thu, xx Sep 2007 xx:xx:xx +0200).
When i disabled the Anti-Spam and pressed enter (in a telnet session to port 25), the SMTP greeting appeared. If anti-spam was enabled, it never appeared. But that didn’t help – Postfix still couldn’t send mails:
postfix/smtp: C65AA88075: conversation with customer.example.com[256.256.256.256] timed out while receiving the initial server greeting
I’ve looked at every setting on both the ZyWALL and the Exchange server, but didn’t find any unusual DNS etc. setting. I even disabled all the DNS lookups done on the Exchange server, but to no avail.
But after upgrading the ZyXEL ZyWALL 5′s firmware to the latest version (V4.02(XD.2)), the problem disappeared. While this wasn’t exactly what i was hoping for, at least the problem was now solved.