I have been dealing with this for several months now, and I am running out of ideas...

  1. Company-A has an internal mailserver working as expected.
  2. Company-B has itīs mail service hosted by its ISP (ISP-B).
  3. EMails from Company-A can not be delivered to the mailserver of ISP-B .(meaning that I know of at least 3 domains hosted by ISP-B to whom mail canīt be delivered).
  4. I can not establish a telnet connection from Company-A to ISP-Bīs smtp server, from any of the systems on c-Aīs LAN.
    It either times out of stays forever waiting for the connection to initiate.
    I can do it from 3rd party IPs of diff ISPs.
    I canīt find someone else using c-Aīs ISP to run a test.
  5. No explanation about why I canīt telnet to the smtp is given.
  6. ISP-B dismisses the issue by simply claiming there are no blockings in his side that could be causing this.
  7. All sorts of tests and verification were made in Company-A, and everything looks good. This includes online tests, RBL checks, rDNS, etc. etc.
  8. All I can think of at this stage, is that ISP-B is blocking us, be it by our IP, IP block, or because of some particular hardware/software combination they might have (re: http://support.microsoft.com/kb/895857). No answers received about that either.
  9. My very last idea is to spoof the IP address and see what happens if we were on a different IP / IP block ????.
    All I need is to find out if the telnet session is prompted properly or even prompted at all...

Suggestions greatly appreciated!
Thanks in advance.