How to block or minimize the email spoofing from the internal or accepted domain?

Dear Exchange Server Expert,

We have in house mail server running Microsoft Exchange Server 2010. currently, some users are always receiving the email spoofing that come from internal domain or accepted domain in exchange. For instance, I am on test.com domain and I receive email spoofing that come from my colleague, amy@test.com domain too.

Currently we are using Symantec cloud as the anti spam server. most of the spam will be blocked but not the email spoofing that using the internal or accepted domains. I think because the content and the sender looks legitimate, that's why the Symantec cloud anti spam has bypass them.

is there any way that we can prevent or minimize this?

Thanks.

Regards,