I have seen few queries on exchange forums about this issue on exchange 2010.
The scenario is,
An external user (e.g. “firstname.lastname@example.org”) is able to send email to all users of the organization but not to a specific user "email@example.com". At the same time some other user from the same external domain (e.g. "firstname.lastname@example.org") can send email to the specific user, "email@example.com".
This scenario happens when you have an edge server present in your messaging infra or the anti-spam agents installed on your hub transport server.
You may look/try one of the below reasons/options to work around or fix this issue (go one by one),
- The user "firstname.lastname@example.org" has added the external user “email@example.com” into his/her blocked senders list of outlook client (sender restriction) by mistake. Remove it and try sending the test email again.
- Disable the Sender Filtering Agent (this is to isolate sender filtering configuration).
- See if there are any transport rules present, which is denying the delivery.
Now little more details on the point 1 & 2, which normally creates these kinds of issues more often. And it is important to understand the reason.
In Exchange Server 2010 there is something called Safelist Aggregation. The Safelist Aggregation refers to the anti-spam functionality shared between Outlook and Exchange. This functionality collects information from Safe Recipients Lists, Safe Senders Lists, Blocked Senders Lists, and contact data that Outlook users configure, and finally make this data available to the anti-spam agents on the server that has the edge transport role is installed.
In Exchange 2010, Safelist Aggregation also replicates the per-recipient Blocked Sender list to Edge Transport server. This results the Sender Filtering agent on the Edge server to block the email from those senders.
Note – This feature will also work on a Hub Transport server where we have the anti-spam agents installed.
Hope this gave you a bit of information about the how Exchange Server 2010 anti-spam agents work with the outlook junk email options.
To know more on Safelist Aggregation, see this TechNet article.