I'm having the strangest time getting a Thunderbird email client to connect to my Exchange 2003 server.

I got the incoming IMAP account set up no problem, and I can receive mail. However sending mail will not work no matter what SMTP settings I enter. After checking the server, the proper settings should be port 25 with no authentication or connection security, which I have entered. I can ping the hostname of the server from the client machine in question. The Thunderbird error message I get is:

"Sending of message failed. The message could not be sent because the connection to SMTP server -hostname omitted- was lost in the middle of the transaction."

So I went to the server and double checked the settings for Exchange's SMTP stuff. I have it correct. I tried to telnet (on the server) to localhost 25. It appears to connect and then disconnect immediately, no message, no nothing. When I telnet to other ports (POP-110 for example) I get proper connection messages and a stable connection. There are no firewalls on either the client or the server. There's a firewall on the network but LAN->LAN traffic is unrestricted.

I can reproduce the Thunderbird error on a second client, and I can't get any client to be able to telnet in.

EDIT: I'm having the same problem with Zimbra Desktop.

Anyone have any ideas?

  • Disable the Connection filter on the properties of the SMTP Virtual Server and try both the Thunderbird and Telnet test again.
    – joeqwerty
    Sep 13, 2012 at 15:15

1 Answer 1


In Exchange System Manager, Org/Servers/ServerName/Protocols/SMTP/ Properties on your Virtual Server.

Access tab.... Authentication - Anonymous should be checked

Connection Control - All except...

Relay (key) - Make sure bottom "allow those that authenticate" is checked. I would also, for grins, add the IP of your client to the lest. At least this way you have something to test with.


  • Thank you! It was the Connection Control that I had missed. For some reason the subnet had been added as a single computer entry (eg. instead of ( and wasn't in the list at all. When I deleted the single entry for the subnet and readded it properly as a group of hosts, everything began to work. I'm not sure if adding is actually necessary, but I added it as well. Sep 14, 2012 at 11:50

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