Hello,
Thanks for contacting our forum. We can consider the several level below:
On the internet level, we need AV software or services, such as EOP or third party anti-spam software.
On mailbox server level, we can enable antispam functionality on Mailbox servers
Refer to:
https://technet.microsoft.com/en-us/library/bb201691(v=exchg.160).aspx
On mailbox level:
https://technet.microsoft.com/en-us/library/bb123559(v=exchg.160).aspx
Hope it helps.
Best Regards,
Jason Chao
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