We have an Exchange server 2010 Environment mixed with Exchange 2003.
There is a domain which is external to where we are not able to send emails.
But if we try to send an email from a GMAIL, Yahoo or Hotmail Account, the recipient receives.
Lets say the domain external.com is sending an email to us at internal.com we are receiving it.
We have Postini also as our external Spam and Filtering Gateway and we see that the messages are getting relayed or R-Injected if tried to send from email@example.com to firstname.lastname@example.org.
I tried checking this issue on the website www.mxtoolbox.com
and found that the logs say that the server at external.com accepts only messages if the SMTP Authentication is enabled.
But as i recall, this option is available only if POP3 accounts are configured to put a tick mark in the account settings to say that the server requires SMTP Authentication.
I am able to telnet to the mx record hostname successfully from the SMTP / Transport servers for external.com
but in Exchange i am not sure, i have tried everything of my knowledge.
Please assist on this issue.
Please note: Earlier we were able to send emails to this domain without any issue.