UBB.threads PHP Forum Software Community Forums UBB.classic UBB.classic 5 Support SMTP mail not working

I've just installed UBB on my site, and it seems to work OK, BUT SMTP is not working. I've typed in the IP of our SMTP server (which allows relays from that address) and I just get the error as shown below. I tested the SMTP server from the machine running UBB using telnet and it works fine.

Any ideas?

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Could not connect to server.
The server does not respond appropriately.. it may not be an SMTP server.
Server Error: ""
Server Error: ""
Server Error: ""
Server Error: ""
Server Error: ""