We are seeing the strangest behavior in email flow.
We have a Sonicwall (TZ210) not doing any anti-spam filtering. All email flow is directed to the Brightmail GW to have spam and nasties filtered out. The valid emails are sent to the Exchange server 2010 (SBS 2011). All spam filtering on Exchange is disabled.
Clients get their emails using Outlook 2007.
The issue: some emails do not get through. A user in domain abc.com, sends an email and even gets a delivery receipt. But recipient never receives the email. Another user in same domain abc.com sends emails without issues to the recipient.
So it is not the domain being filtered out as spam source. Something is randomly (so it seems) blocking emails from certain users.
We are also using Symantec Endpoint on all client computers.
Any help would be appreciated.