Weird SMTP problem


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Hey guys, I wonder if you can help...

At work, we've been having some strange email problems for the past few days. Basically we can't send email. Now we're 99% certain that it's a problem with the Exchange set up, but while we were diagnosing it we've come across something pretty weird.

When you telnet to port 25 of an external SMTP server, the server responds with the normal welcome message (it's id, etc). As soon as you send any data (i.e. by hitting a key), telnet disconnects reporting that the connection has been dropped. This happens when you connect to any smtp server. The servers exhibit the correct behaviour and accept input from telnet sessions that don't originate from within the network.

Ahhh, we think, it must be a problem with the pc we're using to telnet out on. But a completely new pc, installed just for this purpose causes exactly the same error when plugged into the network point. So it must be a router problem between the network point and the smtp server... Apparently not, according to the hardware guys.

And just to top it off, the machines that can't telnet out CAN send email via the smtp server when you use a normal email client. And that's the confusing part, because I can packet snoop the connection between the mail client and the smtp server and it replicates exactly what I'm trying to do on telnet but with no problems. I don't understand how both email could be working, yet the telnet to port 25 not work - they seem to be mutually exclusive.

Ironically, now that we've worked out that the mail problems are exchange, we can sort it. But this is bugging me, because it makes me think there's a bigger problem out there...

Anyway, if anyone has something that will enlighten me, it'll save a lot of hair pulling...

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What's your firewall? ISA Server? Checkpoint?

It's ISA Server. But the server's been completely turned off to test things out. And the fresh pc that we tested it on was entirely outside the firewall.

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