Hi,
According to your description, mail flow can work well when they directly access Exchange server while there is problem with accessing your spam filter server. If I misunderstand your meaning, please feel free to let me know.
If yes, the issue is mostly likely caused by the improper configuration on your spam filter server. As far as I know, we can depend on the Exchange feature to manage spam. For more information, you can refer to the following article:
http://technet.microsoft.com/en-us/library/aa996604(v=exchg.141).aspx
Thanks,
Angela Shi
TechNet Community Support
