We are running a Windows Server 2003 SBS the Exchange server stopped sending email about 10 days ago and at the same time stopped responding with the proper HELO statement when using telnet outside of the network. When I use the command "telnet SBS 25" internally I get the correct HELO statement. When I am outside of the network and telnet to "mail.aeon-development.net 25" I get a blank command window. After about 10 seconds the window displays the command prompt.
I can send email internally and can receive email internal and externally but I cannot send email to an external recipient, and when I run a mail test, (mxtoolbox) the tests say that my mail server cannot be found. PLEAS BE ADVISED: We have redirected all our email to a POP account for now until I can get this cleared up.
My first thought was that our ISP, COX Communications Business Service is blocking port 25, but when I called them they said they were not. My second thought was that our firewall had gone bad so I replace it twice and still get the same results. I have run Microsoft tools (netstat -an) to make sure the server is listening on port 25 and it is. I went so far as to run the setup for mail in SBS Server management hoping that may fix the problem, but it did not.
Any thoughts on how to tell where the HELO statement is being blocked or other suggestions would be appreciated.