antivirus Blocking DHL email?

Dear All,

i need some help from gurus here. we are currently hosting different kind of email service in our network ( Mdaemon, Cpanel, Exchange).

for the last 1 month, user reported not able to receive those DHL system generated email. it happens to all the email account in different platform. i have spend a lot of time looking at this but still cannot resolve. i suspect it could be something blocking from the firewall side.

Anyone have encounter this before?