Here's a very strange problem that caused us a lot of lost time. I was setting up Exhange on a recently installed server for a customer on SBS 2003. Outbound emails worked straight away, inbound didn't arrive.
I checked all the usual things, such as ensuring that the DNS MX record was pointing at the static IP address provided by the ISP. I checked the port forwarding in the router. I checked all settings for the SMTP Virtual Server Protocol against another which was known to work. It all looked fine.
I created a PHP script that would open SMTP socket and send a EHLO to an IP address. I then ran network traces to see the effect of the script. I could see the SMTP (port 25) request arriving at the server and I could see that the server responded to this. But the response never arrived at the far end.
Finally in desperation I took the server and router to my own premises so that I could swap components one by one until I found the culprit. I plugged them in and made a single change to the router so that it would pick up my own broadband account. I ran the PHP script. It worked!
So the culprit was the client's ISP who were blocking inbound SMTP. A quick phone call confirmed this to be the case. But what was strange was that the initial SMTP packet did arrive, suggesting that the problem was at our end. Lesson - always challenge your own assumptions.
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Geographical coverage: AlphaOne Computing Services
provides a range of small business computer support and I.T. services throughout Sussex, including
Brighton, Hove, Shoreham, Worthing, Steyning, Burgess Hill,
Haywards Heath, Crawley, Lewes, Newhaven, Seaford, Uckfield and Eastbourne.
