can not receive attachment email coming from external

Dear all,
Our mail server is Redhat 7.2 sendamil with RAV antivirus.
Currently, We have problem with attachment email.
we can send and receive attachment email among our domain
accounts, and we can aslo send attachment email to external
domain (for instance Hotmail.com)email address and get it,
but we could not receive attachment email coming from
external domain, or sometime we could receive it after
serveral hours, even though it's  marked almost at that
time, for example, I sent one attachment email from
Hotmail.com at 2:00pm, I received it and it was maked at
2:02pm, but why it took several hours before I got it (I
checked with pine or webmail)?

Any suggestion will be appreciated.

Best wishes,

Hailong